Implement tilted concave ambient shadow

Bug: skia:7971
Change-Id: I6a92a5021f53ae10fb35ef355081653e96bc04b0
Reviewed-on: https://skia-review.googlesource.com/134844
Commit-Queue: Jim Van Verth <jvanverth@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
2 files changed