vp9: Return the correct size when decoding a superframe

According to avcodec.h, avcodec_decode_video2 should return the number of
bytes used if a frame was decoded.

The current implementation returns size - used size of all the subframes.
This fixes the VLC's bug https://trac.videolan.org/vlc/ticket/16836.

The superframe is always fully consumed.
Signed-off-by: Diego Biurrun <diego@biurrun.de>

libavcodec/vp9.c

@@ -1251,7 +1251,7 @@ static int vp9_decode_packet(AVCodecContext *avctx, void *frame,
                 data += sz;
                 size -= sz;
             }
-            return size;
+            return avpkt->size;
         }
     }