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Result from luma subsampling is left-shifted by 3. This avoids having to
do it during averaging, in alpha search and when building the
prediction. This change does not alter the bitstream.

Results on Subset1
  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

https://arewecompressedyet.com/?job=cfl-baseline%402017-09-06T17%3A41%3A38.041Z&job=cfl-SubsampleQ3%402017-09-06T17%3A42%3A01.252Z

Change-Id: I6e89eac6496f7c36e46364c9223fbcbca6759032