converted GrSimpleTextureEffect to sksl

Bug: skia:
Change-Id: If556c6baad75f22135f429759feabaaec095b900
Reviewed-on: https://skia-review.googlesource.com/21720
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>
14 files changed