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/impl/rtp_packetizer.cc

@@ -0,0 +1,133 @@
+// Copyright 2019 The Chromium Authors
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "cast/streaming/impl/rtp_packetizer.h"
+
+#include <algorithm>
+#include <limits>
+#include <random>
+
+#include "cast/streaming/impl/packet_util.h"
+#include "platform/api/time.h"
+#include "util/big_endian.h"
+#include "util/integer_division.h"
+#include "util/osp_logging.h"
+
+namespace openscreen::cast {
+
+namespace {
+
+// Returns a random sequence number to start with. The reason for using a random
+// number instead of zero is unclear, but this has existed both in several
+// versions of the Cast Streaming spec and in other implementations for many
+// years.
+uint16_t GenerateRandomSequenceNumberStart() {
+  // Use a statically-allocated generator, instantiated upon first use, and
+  // seeded with the current time tick count. This generator was chosen because
+  // it is light-weight and does not need to produce unguessable (nor
+  // crypto-secure) values.
+  static std::minstd_rand generator(static_cast<std::minstd_rand::result_type>(
+      Clock::now().time_since_epoch().count()));
+
+  return std::uniform_int_distribution<uint16_t>()(generator);
+}
+
+}  // namespace
+
+RtpPacketizer::RtpPacketizer(RtpPayloadType payload_type,
+                             Ssrc sender_ssrc,
+                             int max_packet_size)
+    : payload_type_7bits_(static_cast<uint8_t>(payload_type)),
+      sender_ssrc_(sender_ssrc),
+      max_packet_size_(max_packet_size),
+      sequence_number_(GenerateRandomSequenceNumberStart()) {
+  OSP_CHECK(IsRtpPayloadType(payload_type_7bits_));
+  OSP_CHECK_GT(max_packet_size_, kMaxRtpHeaderSize);
+}
+
+RtpPacketizer::~RtpPacketizer() = default;
+
+ByteBuffer RtpPacketizer::GeneratePacket(const EncryptedFrame& frame,
+                                         FramePacketId packet_id,
+                                         ByteBuffer buffer) {
+  OSP_CHECK_GE(static_cast<int>(buffer.size()), max_packet_size_);
+
+  const int num_packets = ComputeNumberOfPackets(frame);
+  OSP_CHECK_GT(num_packets, 0);
+  OSP_CHECK_LT(int{packet_id}, num_packets);
+  const bool is_last_packet = int{packet_id} == (num_packets - 1);
+
+  // Compute the size of this packet, which is the number of bytes of header
+  // plus the number of bytes of payload. Note that the optional Adaptive
+  // Latency information is only added to the first packet.
+  int packet_size = kBaseRtpHeaderSize;
+  const bool include_adaptive_latency_change =
+      (packet_id == 0 &&
+       frame.new_playout_delay > std::chrono::milliseconds(0));
+  if (include_adaptive_latency_change) {
+    OSP_CHECK_LE(frame.new_playout_delay.count(),
+                 int{std::numeric_limits<uint16_t>::max()});
+    packet_size += kAdaptiveLatencyHeaderSize;
+  }
+  int data_chunk_size = max_payload_size();
+  const int data_chunk_start = data_chunk_size * int{packet_id};
+  if (is_last_packet) {
+    data_chunk_size = static_cast<int>(frame.data.size()) - data_chunk_start;
+  }
+  packet_size += data_chunk_size;
+  OSP_CHECK_LE(packet_size, max_packet_size_);
+  const ByteBuffer packet(buffer.data(), packet_size);
+
+  // RTP Header.
+  AppendField<uint8_t>(kRtpRequiredFirstByte, buffer);
+  AppendField<uint8_t>(
+      (is_last_packet ? kRtpMarkerBitMask : 0) | payload_type_7bits_, buffer);
+  AppendField<uint16_t>(sequence_number_++, buffer);
+  AppendField<uint32_t>(frame.rtp_timestamp.lower_32_bits(), buffer);
+  AppendField<uint32_t>(sender_ssrc_, buffer);
+
+  // Cast Header.
+  AppendField<uint8_t>(
+      ((frame.dependency == EncodedFrame::Dependency::kKeyFrame)
+           ? kRtpKeyFrameBitMask
+           : 0) |
+          kRtpHasReferenceFrameIdBitMask |
+          (include_adaptive_latency_change ? 1 : 0),
+      buffer);
+  AppendField<uint8_t>(frame.frame_id.lower_8_bits(), buffer);
+  AppendField<uint16_t>(packet_id, buffer);
+  AppendField<uint16_t>(num_packets - 1, buffer);
+  AppendField<uint8_t>(frame.referenced_frame_id.lower_8_bits(), buffer);
+
+  // Extension of Cast Header for Adaptive Latency change.
+  if (include_adaptive_latency_change) {
+    AppendField<uint16_t>(
+        (kAdaptiveLatencyRtpExtensionType << kNumExtensionDataSizeFieldBits) |
+            sizeof(uint16_t),
+        buffer);
+    AppendField<uint16_t>(frame.new_playout_delay.count(), buffer);
+  }
+
+  // Copy the encrypted payload data into the packet.
+  auto data_chunk = frame.data.subspan(data_chunk_start, data_chunk_size);
+  std::copy(data_chunk.begin(), data_chunk.end(), buffer.data());
+
+  return packet;
+}
+
+int RtpPacketizer::ComputeNumberOfPackets(const EncryptedFrame& frame) const {
+  // The total number of packets is computed by assuming the payload will be
+  // split-up across as few packets as possible.
+  int num_packets = DividePositivesRoundingUp(
+      static_cast<int>(frame.data.size()), max_payload_size());
+  // Edge case: There must always be at least one packet, even when there are no
+  // payload bytes. Some audio codecs, for example, use zero bytes to represent
+  // a period of silence.
+  num_packets = std::max(1, num_packets);
+
+  // Ensure that the entire range of FramePacketIds can be represented.
+  return num_packets <= int{kMaxAllowedFramePacketId} ? num_packets : -1;
+}
+
+}  // namespace openscreen::cast