Start hooking shaders into SkRasterPipelineBlitter.

Here first just the simplest, constant-color shaders.

BUG=skia:

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=4743

Change-Id: I92c6523660e21a1e2aa353524570230282ba5dfe
Reviewed-on: https://skia-review.googlesource.com/4743
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>
5 files changed