Implement Cast Streaming mirroring, DLNA casting, daemon+GUI, and breadd integration

Builds out the full v1 scope: a vendored+patched openscreen subset for
low-latency Cast Streaming (Mirroring receiver 0F5096E8) alongside the
existing Cast V2/HLS and new DLNA/AVTransport casting paths, breadcastd's
Idle/Casting state machine with a private IPC socket, the breadcast GTK4
popup as a thin IPC client, and bread.cast.*/bread.command.cast.* breadd
integration (device discovery, start/stop, mirroring lifecycle events).
Also adds bakery/systemd/Forgejo CI packaging.

Validated end-to-end against a real Chromecast/Google TV: negotiated
Cast Streaming session, live pipeline playback, and daemon+GUI click-to-cast/
stop through the actual popup.

breadcast-caststream-sys/vendor/openscreen/cast/streaming/public/sender.h

@@ -0,0 +1,173 @@
+// Copyright 2020 The Chromium Authors
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef CAST_STREAMING_PUBLIC_SENDER_H_
+#define CAST_STREAMING_PUBLIC_SENDER_H_
+
+#include <stdint.h>
+
+#include <chrono>
+
+#include "cast/streaming/public/encoded_frame.h"
+#include "cast/streaming/public/frame_id.h"
+#include "cast/streaming/public/session_config.h"
+#include "cast/streaming/rtp_time.h"
+#include "cast/streaming/ssrc.h"
+#include "platform/api/time.h"
+
+namespace openscreen::cast {
+
+// The Cast Streaming Sender, a peer corresponding to some Cast Streaming
+// Receiver at the other end of a network link.
+//
+// The Sender is the peer responsible for enqueuing EncodedFrames for streaming,
+// guaranteeing their delivery to a Receiver, and handling feedback events from
+// a Receiver. Some feedback events are used for managing the Sender's internal
+// queue of in-flight frames, requesting network packet re-transmits, etc.;
+// while others are exposed via the Sender's public interface. For example,
+// sometimes the Receiver signals that it needs a a key frame to resolve a
+// picture loss condition, and the modules upstream of the Sender (e.g., where
+// encoding happens) should call NeedsKeyFrame() to check for, and handle that.
+//
+// There are usually one or two Senders in a streaming session, one for audio
+// and one for video. Both senders work with the same SenderPacketRouter
+// instance to schedule their transmission of packets, and provide the necessary
+// metrics for estimating bandwidth utilization and availability.
+//
+// It is the responsibility of upstream code modules to handle congestion
+// control. With respect to this Sender, that means the media encoding bit rate
+// should be throttled based on network bandwidth availability. This Sender does
+// not do any throttling, only flow-control. In other words, this Sender can
+// only manage its in-flight queue of frames, and if that queue grows too large,
+// it will eventually reject further enqueuing.
+//
+// General usage: A client should check the in-flight media duration frequently
+// to decide when to pause encoding, to avoid wasting system resources on
+// encoding frames that will likely be rejected by the Sender. The client should
+// also frequently call NeedsKeyFrame() and, when this returns true, direct its
+// encoder to produce a key frame soon. Finally, when using EnqueueFrame(), an
+// EncodedFrame struct should be prepared with its frame_id field set to
+// whatever GetNextFrameId() returns. Please see method comments for
+// more-detailed usage info.
+class Sender {
+ public:
+  // Interface for receiving notifications about events of possible interest.
+  class Observer {
+   public:
+    // Called when a frame was canceled, which may occur in the following cases:
+    //  - The Receiver acknowledged successful receipt of the frame.
+    //  - The Receiver decided to skip over the frame (e.g. it was too late).
+    //  - The Sender decided to skip the frame (e.g. OnFrameCanceled() called).
+    //
+    // Note: Frame cancellations may occur out-of-order.
+    virtual void OnFrameCanceled(FrameId frame_id) = 0;
+
+    // Called when a Receiver begins reporting picture loss, and there is no key
+    // frame currently enqueued in the Sender. The application should enqueue a
+    // key frame as soon as possible.
+    //
+    // This acts as a "push" notification, which is useful for immediately
+    // waking up an application that may be waiting for the next capture tick.
+    // For "pull" state checking inside a continuous encoding loop, see
+    // NeedsKeyFrame().
+    virtual void OnPictureLost() = 0;
+
+   protected:
+    virtual ~Observer();
+  };
+
+  // Result codes for EnqueueFrame().
+  enum EnqueueFrameResult {
+    // The frame has been queued for sending.
+    OK,
+
+    // The frame's payload was too large.
+    PAYLOAD_TOO_LARGE,
+
+    // The span of FrameIds is too large.
+    REACHED_ID_SPAN_LIMIT,
+
+    // Too-large a media duration is in-flight.
+    MAX_DURATION_IN_FLIGHT,
+  };
+
+  virtual ~Sender();
+
+  // The session configuration for this sender. The configuration is generated
+  // from the offer/answer exchange, and includes critical information like the
+  // RTP timebase, SSRCs for sending and receiving, and the AES configuration.
+  virtual const SessionConfig& config() const = 0;
+
+  // Sets an observer for receiving notifications. Call with nullptr to stop
+  // observing.
+  virtual void SetObserver(Observer* observer) = 0;
+
+  // Returns the number of frames currently in-flight. This is only meant to be
+  // informative. Clients should use GetInFlightMediaDuration() to make
+  // throttling decisions.
+  virtual size_t GetInFlightFrameCount() const = 0;
+
+  // Returns the total media duration of the frames currently in-flight,
+  // assuming the next not-yet-enqueued frame will have the given RTP timestamp.
+  // For a better user experience, the result should be compared to
+  // GetMaxInFlightMediaDuration(), and media encoding should be throttled down
+  // before additional EnqueueFrame() calls would cause this to reach the
+  // current maximum limit.
+  virtual Clock::duration GetInFlightMediaDuration(
+      RtpTimeTicks next_frame_rtp_timestamp) const = 0;
+
+  // Return the maximum acceptable in-flight media duration, given the current
+  // target playout delay setting and end-to-end network/system conditions.
+  virtual Clock::duration GetMaxInFlightMediaDuration() const = 0;
+
+  // Returns true if the Receiver requires a key frame. Note that this will
+  // return true until a key frame is accepted by EnqueueFrame(). Thus, when
+  // encoding is pipelined, care should be taken to instruct the encoder to
+  // produce just ONE forced key frame.
+  //
+  // This acts as a stateful "pull" check, which is useful for an encoder loop
+  // to poll right before processing the next image. For "push" notifications
+  // to wake up an idle application, see Observer::OnPictureLost().
+  virtual bool NeedsKeyFrame() const = 0;
+
+  // Returns the next FrameId, the one after the frame enqueued by the last call
+  // to EnqueueFrame(). Note that the next call to EnqueueFrame() assumes this
+  // frame ID be used.
+  virtual FrameId GetNextFrameId() const = 0;
+
+  // Get the current round trip time, defined as the total time between when the
+  // sender report is sent and the receiver report is received. This value is
+  // updated with each receiver report using a weighted moving average of 1/8
+  // for the new value and 7/8 for the previous value. Will be set to
+  // Clock::duration::zero() if no reports have been received yet.
+  // TODO(crbug.com/498036656): move to a more modern approach for estimating
+  // bandwidth.
+  virtual Clock::duration GetCurrentRoundTripTime() const = 0;
+
+  // Enqueues the given `frame` for sending as soon as possible. Returns OK if
+  // the frame is accepted, and some time later Observer::OnFrameCanceled() will
+  // be called once it is no longer in-flight.
+  //
+  // All fields of the `frame` must be set to valid values: the `frame_id` must
+  // be the same as GetNextFrameId(); both the `rtp_timestamp` and
+  // `reference_time` fields must be monotonically increasing relative to the
+  // prior frame; and the frame's `data` pointer must be set.
+  [[nodiscard]] virtual EnqueueFrameResult EnqueueFrame(
+      const EncodedFrame& frame) = 0;
+
+  // Causes all pending operations to discard data when they are processed
+  // later. This will notify observers by invoking OnFrameCanceled() for each
+  // canceled frame.
+  virtual void CancelInFlightData() = 0;
+
+  // May be called by the consumer to report that a frame has been dropped. This
+  // is used to report drop statistics to the sender's statistics collector.
+  virtual void ReportFrameDropEvent(FrameId frame_id,
+                                    RtpTimeTicks rtp_timestamp,
+                                    Clock::time_point drop_time) = 0;
+};
+
+}  // namespace openscreen::cast
+
+#endif  // CAST_STREAMING_PUBLIC_SENDER_H_