A solid background can't reveal whether a surface that's supposed to be
translucent (breadbox/breadclip/breadsearch's full-screen overlay
windows, breadbar's alpha-blended notification/OSD surfaces) is actually
compositing as translucent — a flat color showing through a flat color
still just looks flat. Generates a diagonal rainbow gradient (full hue
sweep, via the `image` crate) sized exactly to the capture canvas and
sets it as Sway's output background instead.
Needed installing swaybg (the external helper Sway's `output ... bg`
config directive shells out to) — without it the bg command silently
no-ops and the canvas stays black, which is why the first attempt at
this looked identical to the old flat-color version.
e3df426996