Separate PipelineStage's sampleChild into two callbacks.

We now have `sampleShader` and `sampleColorFilter`. This CL prepares us
for the addition of a third sampleable type, SkBlender, with a unique
signature (two colors, no coordinates).

Change-Id: I77ba9ba94a1c447d0ef10f5acb5ffe9b9395cfe1
Bug: skia:12257
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/431697
Commit-Queue: John Stiles <johnstiles@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
5 files changed