Refines variance based partitioning search

Instead of using source variance, this patch uses variance of the
frame difference between the source and the current frame to make
fixed size partition decisions. Also disables adjusting partitioning
if variance based or fixed size partitioning is used.

The latter change improves the speed substantially for speed 6, so
that speed 7 is now less than 3x the speed of speed 6. But speed
6 is 48% better in psnr on the rtc set compared to speed 7.

As compared to speed 5,
speed 6 is -37% in psnr at about 2.5x the speed,
speed 7 is -55% in psnr at about 7x the speed.

Change-Id: If61d80431d3e04ed304ac05832e773cdb2c0a578