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