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/src/trait/frame_source.h

@@ -0,0 +1,38 @@
+#ifndef SC_FRAME_SOURCE_H
+#define SC_FRAME_SOURCE_H
+
+#include "common.h"
+
+#include "frame_sink.h"
+
+#define SC_FRAME_SOURCE_MAX_SINKS 2
+
+/**
+ * Frame source trait
+ *
+ * Component able to send AVFrames should implement this trait.
+ */
+struct sc_frame_source {
+    struct sc_frame_sink *sinks[SC_FRAME_SOURCE_MAX_SINKS];
+    unsigned sink_count;
+};
+
+void
+sc_frame_source_init(struct sc_frame_source *source);
+
+void
+sc_frame_source_add_sink(struct sc_frame_source *source,
+                         struct sc_frame_sink *sink);
+
+bool
+sc_frame_source_sinks_open(struct sc_frame_source *source,
+                           const AVCodecContext *ctx);
+
+void
+sc_frame_source_sinks_close(struct sc_frame_source *source);
+
+bool
+sc_frame_source_sinks_push(struct sc_frame_source *source,
+                           const AVFrame *frame);
+
+#endif