Expose screen as frame sink

Make screen implement the frame sink trait.

This will allow the decoder to push frames without depending on the
concrete sink type.

app/src/screen.h

@@ -9,10 +9,13 @@
 
 #include "coords.h"
 #include "opengl.h"
+#include "trait/frame_sink.h"
 
 struct video_buffer;
 
 struct screen {
+    struct sc_frame_sink frame_sink; // frame sink trait
+
     struct video_buffer *vb;
     struct fps_counter *fps_counter;