Cache ambient and spot shadows at a canonical position

Change-Id: I1f80931513f7d2268b358fb38c86dd331f32f064
Reviewed-on: https://skia-review.googlesource.com/17394
Commit-Queue: Jim Van Verth <jvanverth@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
3 files changed