Force_split on 16x16 blocks in variance partition.

Force split on 16x16 block (to 8x8) based on the minmax over the 8x8 sub-blocks.

Also increase variance threshold for 32x32, and add exit condiiton in choose_partition
(with very safe threshold) based on sad used to select reference frame.

Some visual improvement near moving boundaries.
Average gain in psnr/ssim: ~0.6%, some clips go up ~1 or 2%.
Encoding time increase (due to more 8x8 blocks) from ~1-4%, depending on clip.

Change-Id: I4759bb181251ac41517cd45e326ce2997dadb577