vp6: Support cropping to AVCodecContext.width/height

In these cases, there is no extradata but only the properly set
width/height values by the demuxer.

This makes sure VP6 in F4V files is cropped properly.

This is similar to what is done for H264 for letting the container
width/height override what's in the bitstream, since 30f51509.

Signed-off-by: Martin Storsjö <martin@martin.st>