Add working color space shader

This lets an SkShader operate in a particular SkColorSpace. Most useful
for SkSL, but also works for things like SkShaders::Blend.

Bug: b/299670828
Change-Id: Ifb250a600540f276c5e16b7813b988fc0134dea1
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/755517
Commit-Queue: Brian Osman <brianosman@google.com>
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
12 files changed