Add ShaderType::kSolidColor and make Combinations work w/ PaintParams

This is split out of Michael's CL:
https://skia-review.googlesource.com/c/skia/+/468459

Bug: skia:12466
Change-Id: Ic38ee577553cf2c0d4da8130e80a11422fa2b92e
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/472177
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
11 files changed