- get rid of an unnecessary seek that was messing up either mkv or vc1 decoding.

 - get rid of an unnecessary seek that was messing up either mkv or vc1 decoding.
 - switch av_seek_frame to zero back to time-based rather than byte-based since time-based screws up mkv & mp4 while time-based works for everything but vc1.
 - since ffmpeg doesn't correctly indicate key frames in vc1 look for them ourselves so that vc1 previews & cropping will work.

git-svn-id: svn://svn.handbrake.fr/HandBrake/trunk@1927 b64f7644-9d1e-0410-96f1-a4d463321fa5