Fill in gamut-transformation TODOs for software pipeline.

This adds support for paint colors, color shaders, and mode color filters (everything that the software pipeline supports today).

BUG=skia:

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=4781

Change-Id: I6e5da6d0dba03fbc82ecaa233ce8c727e7ce17b3
Reviewed-on: https://skia-review.googlesource.com/4781
Reviewed-by: Brian Osman <brianosman@google.com>