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 30f515091.

Signed-off-by: Martin Storsjö <martin@martin.st>
flvenc: Don't pretend to support muxing "plain" VP6

The plain VP6 format is vertically flipped compared to VP6F/VP6A.
Support for the plain VP6 format was added in 09d8c0ae831 (which
also introduced support for muxing VP6F properly in general).

Signed-off-by: Martin Storsjö <martin@martin.st>
flvenc: Support muxing VP6A as well

Handle it in the same way as VP6F, except for the codec tag.

Signed-off-by: Martin Storsjö <martin@martin.st>
flvenc: Write proper cropping for VP6 even if there's no extradata

This keeps cropping when remuxing from F4V to FLV.

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