Extract current video_buffer to frame_buffer

The current video buffer only stores one pending frame.

In order to add a new buffering feature, move this part to a separate
"frame buffer". Keep the video_buffer, which currently delegates all its
calls to the frame_buffer.

app/src/frame_buffer.h

@@ -0,0 +1,44 @@
+#ifndef SC_FRAME_BUFFER_H
+#define SC_FRAME_BUFFER_H
+
+#include "common.h"
+
+#include <stdbool.h>
+
+#include "util/thread.h"
+
+// forward declarations
+typedef struct AVFrame AVFrame;
+
+/**
+ * A frame buffer holds 1 pending frame, which is the last frame received from
+ * the producer (typically, the decoder).
+ *
+ * If a pending frame has not been consumed when the producer pushes a new
+ * frame, then it is lost. The intent is to always provide access to the very
+ * last frame to minimize latency.
+ */
+
+struct sc_frame_buffer {
+    AVFrame *pending_frame;
+    AVFrame *tmp_frame; // To preserve the pending frame on error
+
+    sc_mutex mutex;
+
+    bool pending_frame_consumed;
+};
+
+bool
+sc_frame_buffer_init(struct sc_frame_buffer *fb);
+
+void
+sc_frame_buffer_destroy(struct sc_frame_buffer *fb);
+
+bool
+sc_frame_buffer_push(struct sc_frame_buffer *fb, const AVFrame *frame,
+                     bool *skipped);
+
+void
+sc_frame_buffer_consume(struct sc_frame_buffer *fb, AVFrame *dst);
+
+#endif