set AVCodecContext.width/height to the picture width/height instead of the one stored...

set AVCodecContext.width/height to the picture width/height instead of the one stored in the bitstream (that only matters if lowres!=0)

git-svn-id: file:///var/local/repositories/ffmpeg/trunk@3518 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b