Gamut transformation of the paint color in Ganesh

Conversion from sRGB to destination gamut is going to be very common,
so I'm caching that xform (if there is one) on the draw context.

Results verified in the gamut GM (two more boxes correct).

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2330553003

Review-Url: https://codereview.chromium.org/2330553003
5 files changed