Add per-frame SSIM/PSNR stats to tools/tiny_ssim.

Adds an optional output framestats.csv file that prints comparions
per-frame instead of averaged over the entire clip. It prints
per-channel and combined metrics for SSIM and PSNR.

Change-Id: Id28dfade27bc5775b59a9d83cfe8b37d1d52b686
1 file changed