Notify new frames via callbacks

Currently, a frame is available to the consumer as soon as it is pushed
by the producer (which can detect if the previous frame is skipped).

Notify the new frames (and frame skipped) via callbacks instead.

This paves the way to add (optional) buffering, which will introduce a
delay between the time when the frame is produced and the time it is
available to be consumed.

app/src/video_buffer.c

@@ -7,8 +7,20 @@
 #include "util/log.h"
 
 bool
-sc_video_buffer_init(struct sc_video_buffer *vb) {
-    return sc_frame_buffer_init(&vb->fb);
+sc_video_buffer_init(struct sc_video_buffer *vb,
+                     const struct sc_video_buffer_callbacks *cbs,
+                     void *cbs_userdata) {
+    bool ok = sc_frame_buffer_init(&vb->fb);
+    if (!ok) {
+        return false;
+    }
+
+    assert(cbs);
+    assert(cbs->on_new_frame);
+
+    vb->cbs = cbs;
+    vb->cbs_userdata = cbs_userdata;
+    return true;
 }
 
 void
@@ -17,9 +29,15 @@ sc_video_buffer_destroy(struct sc_video_buffer *vb) {
 }
 
 bool
-sc_video_buffer_push(struct sc_video_buffer *vb, const AVFrame *frame,
-                  bool *previous_frame_skipped) {
-    return sc_frame_buffer_push(&vb->fb, frame, previous_frame_skipped);
+sc_video_buffer_push(struct sc_video_buffer *vb, const AVFrame *frame) {
+    bool previous_skipped;
+    bool ok = sc_frame_buffer_push(&vb->fb, frame, &previous_skipped);
+    if (!ok) {
+        return false;
+    }
+
+    vb->cbs->on_new_frame(vb, previous_skipped, vb->cbs_userdata);
+    return true;
 }
 
 void