Add shader that clamps the coordinates used to sample another shader.

Not implemented yet on Graphite.

Bug: chromium:1341445

Change-Id: Iabbbe06d8b540eda05933b3500d5dbc08a48713f
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/637515
Commit-Queue: Brian Salomon <bsalomon@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
13 files changed