In order to address hardware concerns regarding luma averaging,
this patch caps the maximum length of the sides of the luma
averaging area to 32.

This proposal was accepted by the hardware working group on
November 20th 2017.

Regression on Subset1:
  PSNR | PSNR Cb | PSNR Cr | PSNR HVS |    SSIM | MS SSIM | CIEDE 2000
0.0078 |  0.0572 | -0.0705 |  -0.0272 | -0.0202 | -0.0391 |     0.0375

Change-Id: I875a6f2114df4d857ed66c4690ee08da2df426e4