Extract sc_delay_buffer

A video buffer had 2 responsibilities:
 - handle the frame delaying mechanism (queuing packets and pushing them
   after the expected delay);
 - keep only the most recent frame (using a sc_frame_buffer).

In order to be able to reuse only the frame delaying mechanism, extract
it to a separate component, sc_delay_buffer.

app/src/delay_buffer.h

@@ -0,0 +1,69 @@
+#ifndef SC_DELAY_BUFFER_H
+#define SC_DELAY_BUFFER_H
+
+#include "common.h"
+
+#include <stdbool.h>
+
+#include "clock.h"
+#include "util/thread.h"
+#include "util/tick.h"
+#include "util/vecdeque.h"
+
+// forward declarations
+typedef struct AVFrame AVFrame;
+
+struct sc_delayed_frame {
+    AVFrame *frame;
+#ifndef NDEBUG
+    sc_tick push_date;
+#endif
+};
+
+struct sc_delayed_frame_queue SC_VECDEQUE(struct sc_delayed_frame);
+
+struct sc_delay_buffer {
+    sc_tick delay;
+
+    // only if delay > 0
+    struct {
+        sc_thread thread;
+        sc_mutex mutex;
+        sc_cond queue_cond;
+        sc_cond wait_cond;
+
+        struct sc_clock clock;
+        struct sc_delayed_frame_queue queue;
+        bool stopped;
+    } b; // buffering
+
+    const struct sc_delay_buffer_callbacks *cbs;
+    void *cbs_userdata;
+};
+
+struct sc_delay_buffer_callbacks {
+    bool (*on_new_frame)(struct sc_delay_buffer *db, const AVFrame *frame,
+                         void *userdata);
+};
+
+bool
+sc_delay_buffer_init(struct sc_delay_buffer *db, sc_tick delay,
+                     const struct sc_delay_buffer_callbacks *cbs,
+                     void *cbs_userdata);
+
+bool
+sc_delay_buffer_start(struct sc_delay_buffer *db);
+
+void
+sc_delay_buffer_stop(struct sc_delay_buffer *db);
+
+void
+sc_delay_buffer_join(struct sc_delay_buffer *db);
+
+void
+sc_delay_buffer_destroy(struct sc_delay_buffer *db);
+
+bool
+sc_delay_buffer_push(struct sc_delay_buffer *db, const AVFrame *frame);
+
+#endif