Replace SkShader_Lerp with a runtime shader

Change-Id: If3bd5cfb3fb04c747f913387a6245b35b8d5fc4b
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/402398
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
3 files changed