BITSTREAM: Handle transform size and motion vectors more logically for non-420.

BITSTREAM: Handle transform size and motion vectors more logically for non-420.

This breaks the profile 1 bitstream.

Don't force non420 uv transform size to 1/4 y size. In the 4:2:0 case the
chroma corresponding to a luma block is 1/4 its size. In the 4:4:4 case
chroma and luma planes are the same size. Disallowing larger transforms
can result in a loss of compression efficiency and is inconsistent.

For sub-8x8 blocks only average corresponding motion vectors.

4:2:0 and profile 0 behavior remains unchanged.

Change-Id: I560ae07183012c6734dd1860ea54ed6f62f3cae8