Since the scaled luma can be negative, ROUND_POWER_OF_TWO_SIGNED must be used.
This changes the behavior from rounding toward -infinity to rounding towards 0.

Results for Subset1 (compared with 35545dd5 with CfL enabled)
  PSNR | PSNR Cb | PSNR Cr | PSNR HVS |    SSIM | MS SSIM | CIEDE 2000
0.0082 | -0.1061 | -0.0119 |  -0.0126 | -0.0011 | -0.0121 |     0.0094

Change-Id: Ie7258a17a199368339d4794fba6b5916e607c95b