Add intermediate frame in video buffer

There were only two frames simultaneously:
 - one used by the decoder;
 - one used by the renderer.

When the decoder finished decoding a frame, it swapped it with the
rendering frame.

Adding a third frame provides several benefits:
 - the decoder do not have to wait for the renderer to release the
   mutex;
 - it simplifies the video_buffer API;
 - it makes the rendering frame valid until the next call to
   video_buffer_take_rendering_frame(), which will be useful for
   swscaling on window resize.

app/src/video_buffer.h

@@ -11,14 +11,35 @@
 // forward declarations
 typedef struct AVFrame AVFrame;
 
+/**
+ * There are 3 frames in memory:
+ *  - one frame is held by the decoder (decoding_frame)
+ *  - one frame is held by the renderer (rendering_frame)
+ *  - one frame is shared between the decoder and the renderer (pending_frame)
+ *
+ * The decoder decodes a packet into the decoding_frame (it may takes time).
+ *
+ * Once the frame is decoded, it calls video_buffer_offer_decoded_frame(),
+ * which swaps the decoding and pending frames.
+ *
+ * When the renderer is notified that a new frame is available, it calls
+ * video_buffer_take_rendering_frame() to retrieve it, which swaps the pending
+ * and rendering frames. The frame is valid until the next call, without
+ * blocking the decoder.
+ */
+
 struct video_buffer {
     AVFrame *decoding_frame;
+    AVFrame *pending_frame;
     AVFrame *rendering_frame;
+
     sc_mutex mutex;
     bool render_expired_frames;
     bool interrupted;
-    sc_cond rendering_frame_consumed_cond;
-    bool rendering_frame_consumed;
+
+    sc_cond pending_frame_consumed_cond;
+    bool pending_frame_consumed;
+
     struct fps_counter *fps_counter;
 };
 
@@ -30,18 +51,15 @@ void
 video_buffer_destroy(struct video_buffer *vb);
 
 // set the decoded frame as ready for rendering
-// this function locks vb->mutex during its execution
 // the output flag is set to report whether the previous frame has been skipped
 void
 video_buffer_offer_decoded_frame(struct video_buffer *vb,
                                  bool *previous_frame_skipped);
 
 // mark the rendering frame as consumed and return it
-// MUST be called with vb->mutex locked!!!
-// the caller is expected to render the returned frame to some texture before
-// unlocking vb->mutex
+// the frame is valid until the next call to this function
 const AVFrame *
-video_buffer_consume_rendered_frame(struct video_buffer *vb);
+video_buffer_take_rendering_frame(struct video_buffer *vb);
 
 // wake up and avoid any blocking call
 void