There were two places where invisible pixels are included
when calculating distortion with dist-8x8 is turned on.
Those are bugs and fixed by this commit.

1) When av1_dist_8x8() is called in dist_8x8_sub8x8_txfm_rd()
- New assert() are also added to detect any failures in the future.

2) Inside av1_dist_8x8() and av1_dist_8x8_diff(), with tune-metric == PSNR.

Change-Id: I078d1feb3bb010c0e34041640bb69a400125817a