sksl fragment processor support

Bug: skia:
Change-Id: Ia3b0305c2b0c78074303831f628fb01852b90d34
Reviewed-on: https://skia-review.googlesource.com/17843
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
Reviewed-by: Ben Wagner <benjaminwagner@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>
54 files changed