Added DSL sk_SampleCoord var

Change-Id: I539a288765243b2b9d994240c7917b036590f49d
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/370916
Reviewed-by: John Stiles <johnstiles@google.com>
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
Auto-Submit: Ethan Nicholas <ethannicholas@google.com>
4 files changed