Just using with composeshader for now, plan to try that sort of generalization
for colorfilters and imagefilters in follow-on cls.
Bug: skia:
Change-Id: Ic9650b8ea6f6278e6bfd657e90befbf9e71f383c
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/198823
Commit-Queue: Mike Reed <reed@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>
