Introduce frame source trait

There was a frame sink trait, implemented by components able to receive
AVFrames, but each frame source had to manually send to frame sinks.

In order to mutualise sink management, add a frame sink trait.

app/meson.build

@@ -31,6 +31,7 @@ src = [
     'src/server.c',
     'src/version.c',
     'src/video_buffer.c',
+    'src/trait/frame_source.c',
     'src/trait/packet_source.c',
     'src/util/acksync.c',
     'src/util/average.c',