This is more precise than the dist functions it replaces.

Results on Subset1 (compared with previous commit with CfL enabled)
  PSNR | PSNR Cb | PSNR Cr | PSNR HVS |   SSIM | MS SSIM | CIEDE 2000
0.0634 | -0.9188 | -0.9429 |   0.0609 | 0.0722 |  0.0593 |    -0.3226

Change-Id: I955a7d7eceea50482edb40b0d1041b300e3c9042