Initial clipShader implementation for SkClipStack and GPU

Change-Id: I0af800900a7fbd9d16af0058ee0754358ebc3875
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/293562
Commit-Queue: Michael Ludwig <michaelludwig@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
18 files changed