Add geometric implementation for ambient shadows

BUG=skia:6119

Change-Id: I3140522f223c35fc059a33b593064897485dff7c
Reviewed-on: https://skia-review.googlesource.com/7273
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Jim Van Verth <jvanverth@google.com>
6 files changed