use coverage modes instead of blend modes

Bug: skia:
Change-Id: Ib3aa0137644358173ea4087693f33dbc2118c6d2
Reviewed-on: https://skia-review.googlesource.com/99661
Commit-Queue: Mike Reed <reed@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
5 files changed