Refactor stencil clip mask generation into helper

Change-Id: If3dc80efde3b44e87ba8e7af3a258896ec5e78e6
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/288977
Commit-Queue: Michael Ludwig <michaelludwig@google.com>
Reviewed-by: Chris Dalton <csmartdalton@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
8 files changed