LibVideo: Allow the VP9 decoder to queue multiple frames

Frames will now be queued for retrieval by the user of the decoder.
When the end of the current queue is reached, a DecoderError of
category NeedsMoreInput will be emitted, allowing the caller to react
by displaying what was previously retrieved for sending more samples.

Userland/Libraries/LibVideo/PlaybackManager.h

@@ -124,7 +124,8 @@ private:
     bool prepare_next_frame();
     void update_presented_frame();
 
-    // Runs off the main thread
+    // May run off the main thread
+    void post_decoder_error(DecoderError error);
     bool decode_and_queue_one_sample();
     void on_decode_timer();