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 frame to sinks.

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

app/src/trait/frame_source.c

@@ -0,0 +1,59 @@
+#include "frame_source.h"
+
+void
+sc_frame_source_init(struct sc_frame_source *source) {
+    source->sink_count = 0;
+}
+
+void
+sc_frame_source_add_sink(struct sc_frame_source *source,
+                         struct sc_frame_sink *sink) {
+    assert(source->sink_count < SC_FRAME_SOURCE_MAX_SINKS);
+    assert(sink);
+    assert(sink->ops);
+    source->sinks[source->sink_count++] = sink;
+}
+
+static void
+sc_frame_source_sinks_close_firsts(struct sc_frame_source *source,
+                                    unsigned count) {
+    while (count) {
+        struct sc_frame_sink *sink = source->sinks[--count];
+        sink->ops->close(sink);
+    }
+}
+
+bool
+sc_frame_source_sinks_open(struct sc_frame_source *source,
+                           const AVCodecContext *ctx) {
+    assert(source->sink_count);
+    for (unsigned i = 0; i < source->sink_count; ++i) {
+        struct sc_frame_sink *sink = source->sinks[i];
+        if (!sink->ops->open(sink, ctx)) {
+            sc_frame_source_sinks_close_firsts(source, i);
+            return false;
+        }
+    }
+
+    return true;
+}
+
+void
+sc_frame_source_sinks_close(struct sc_frame_source *source) {
+    assert(source->sink_count);
+    sc_frame_source_sinks_close_firsts(source, source->sink_count);
+}
+
+bool
+sc_frame_source_sinks_push(struct sc_frame_source *source,
+                            const AVFrame *frame) {
+    assert(source->sink_count);
+    for (unsigned i = 0; i < source->sink_count; ++i) {
+        struct sc_frame_sink *sink = source->sinks[i];
+        if (!sink->ops->push(sink, frame)) {
+            return false;
+        }
+    }
+
+    return true;
+}