Calculate SSIM over both reconstruction as well as postproc buffer.

Calculate SSIM over both reconstruction as well as postproc buffer.

We used to calculate SSIM only over the postproc buffer, whereas we
calculate PSNR for both. Compared to postproc-SSIM, this is about 0.3%
higher for derf, 1.4% lower for hd and 0.5% lower for stdhd, although
it is highly variable on a per-clip basis.

Change-Id: I8dd491f0f5b4201dedfb15d288c854d5d4caa10f