Better encapsulate Blend, Color and Empty shaders


Bug: skia:13438
Change-Id: Id1b7e6759144820f200040616deb3a120d809765
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/554405
Commit-Queue: Robert Phillips <robertphillips@google.com>
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
20 files changed