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>