Add geometric implementation for ambient shadows

Original: https://skia-review.googlesource.com/7273

BUG=skia:6119

Change-Id: Ie7debd7727768f55dafad922a2b8b9fd3a638fda
Reviewed-on: https://skia-review.googlesource.com/7500
Commit-Queue: Jim Van Verth <jvanverth@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
6 files changed