Abstracted diffuse color map out of SkLightingShaderImpl into a generic SkShader

Will not run until after landing https://codereview.chromium.org/2106893003/

This CL's base is the CL for the addition of NormalSource to the API: https://codereview.chromium.org/2063793002/

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2062703003

Committed: https://skia.googlesource.com/skia/+/3f58cd0eb8bf4499c4f179aa6111d7e005809df8
Review-Url: https://codereview.chromium.org/2062703003
2 files changed