Avoids blurring chroma for 4:2:0

PSNR YCbCr:      0.03%     -0.31%     -0.29%
   PSNRHVS:      0.02%
      SSIM:      0.03%
    MSSSIM:      0.02%
 CIEDE2000:      0.01%

Change-Id: If744fb902b5f24404479def22b9ca8a19baec722