GM: run mode comparisons for all raster configs.

GM: run mode comparisons for all raster configs.

GM works like this today:
  - Test all enabled configs directly
  - Render into 8888
  - Test all enabled modes against that 8888.

This change makes it do this:
  - Test all enabled configs directly
    - for each config, if it's raster, test all enabled modes against its output.

Upshot is, we now check that picture draws identically to raster for 565 too, not just 8888.  optimizations-565 is correctly failing.

BUG=skia:1994
R=epoger@google.com, reed@google.com

Author: mtklein@google.com

Review URL: https://codereview.chromium.org/129203002

git-svn-id: http://skia.googlecode.com/svn/trunk@13050 2bbb7eff-a529-9590-31e7-b0007b416f81