ffmpeg: restore invalid vda_frame definition

Upstream has it this way already, no idea how this ended up changed.
dvdplayer: support vda decoding using ffmpeg hwaccel infrastructure

This removes the requirment that width/height have to be known before
hand, as well as avoid need to extract extradata before hand.
Merge pull request #2847 from elupus/vda_ffmpeg

Vda ffmpeg