Improve the gamma GM significantly

Improve the gamma GM significantly

- Switch to portable typeface, to avoid per-platform diffs in the labels
- Stretch out the image slightly, to avoid overlap with larger font
- Fix several tests that were no longer matching ground-truth. We treat
  SkColor as sRGB, so 0x7F is incorrect, for example. Several of the
  XferMode tests had similar math errors now. Computed new values, and
  verified that they work as expected.
- Removed a couple tests that were mixing color and alpha in funny ways

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2288053003

Review-Url: https://codereview.chromium.org/2288053003