Expose decoder as packet sink

Make decoder implement the packet sink trait.

This will allow the stream to push packets without depending on the
concrete sink type.

app/src/decoder.h

@@ -3,12 +3,16 @@
 
 #include "common.h"
 
+#include "trait/packet_sink.h"
+
 #include <stdbool.h>
 #include <libavformat/avformat.h>
 
 struct video_buffer;
 
 struct decoder {
+    struct sc_packet_sink packet_sink; // packet sink trait
+
     struct video_buffer *video_buffer;
 
     AVCodecContext *codec_ctx;