Fix compilation with --disable-everything --enable-demuxer=matroska.

Fixes ticket #3693.
avcodec/snow: fix 1 pixel block sizes

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avutil/frame: frame_copy_video: support copying from a smaller to a larger frame

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avcodec/asvenc: fix encoding dimensions %16 != 0

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>