Add frame sink trait

This trait will allow to abstract the concrete sink types from the frame
producer (decoder.c).

app/src/trait/frame_sink.h

@@ -0,0 +1,26 @@
+#ifndef SC_FRAME_SINK
+#define SC_FRAME_SINK
+
+#include "common.h"
+
+#include <assert.h>
+#include <stdbool.h>
+
+typedef struct AVFrame AVFrame;
+
+/**
+ * Frame sink trait.
+ *
+ * Component able to receive AVFrames should implement this trait.
+ */
+struct sc_frame_sink {
+    const struct sc_frame_sink_ops *ops;
+};
+
+struct sc_frame_sink_ops {
+    bool (*open)(struct sc_frame_sink *sink);
+    void (*close)(struct sc_frame_sink *sink);
+    bool (*push)(struct sc_frame_sink *sink, const AVFrame *frame);
+};
+
+#endif