rewrote GrAlphaThresholdFragmentProcessor in sksl

Bug: skia:
Change-Id: I641b206fc3bc19ac190ad94ee755ab9e1caab9b3
Reviewed-on: https://skia-review.googlesource.com/21341
Reviewed-by: Jim Van Verth <jvanverth@google.com>
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
11 files changed