This change does not impact the bitstream, it changes how to distortion
is summed when evaluating alpha. The sum is still taken over the entire
partition. However, instead of iterating over the entire surface all at
once, CfL now iterates over each transform block. This is in light of
future work to compute alpha over transform blocks and not prediction
blocks.

Results on Subset1 (compared to 9c6f8547 with CfL)

  PSNR | PSNR Cb | PSNR Cr | PSNR HVS |   SSIM | MS SSIM | CIEDE 2000
0.0000 |  0.0000 |  0.0000 |   0.0000 | 0.0000 |  0.0000 |     0.0000

Change-Id: Ic7b72201d29ad6b2527748e35b212bec515e3bdb