Add color space xform to GrAlphaThresholdFragmentProcessor

Fix imagealphathreshold_surface GM to test gamut conversion.

BUG=skia:

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2867

Change-Id: Id9aaebe72d1dadc613ef1a8d17d066b51049300f
Reviewed-on: https://skia-review.googlesource.com/2867
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
4 files changed