SkPDF: SkPDFMakeShader takes the paint color.

This allows alpha blending and also alpha shaders with color blended in.

fixes GMs:  composeshader_alpha, composeshader_bitmap

Change-Id: I3ab9cbef216f7733798d2e29541b4211c627dab2
Reviewed-on: https://skia-review.googlesource.com/24760
Commit-Queue: Hal Canary <halcanary@google.com>
Reviewed-by: Ben Wagner <bungeman@google.com>
4 files changed