This commit adds the computation of fastSSIM for highbitdepth build,
it also modifies the hbdmetric test to be more generic and applicable
for fastSSIM.

The 255 used for calculating ssim constants c1 and c2 is not exactly
scaled by 4x and 16x to 1023 and 4095, therefore requries the metric
test to have a thresold more tolerant than 0, currently at 0.03dB.

Change-Id: I631829da7773de400e77fc36004156e5e126c7e0