This increases the slice size to 64 pixels, due to having to decode an
entire chroma superblock row per slice.

This can be up to 6% slower depending on clip and CPU, but is necessary
for future optimizations that gain significantly more than was lost.

git-svn-id: file:///var/local/repositories/ffmpeg/trunk@22189 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b