Split ShadowMaskFilter into separate ambient and spot mask filters

This does not change the public API.

BUG=skia:6119

Change-Id: Ibdcd2f8611bc2eec332d8a65e5d51246b89a0a90
Reviewed-on: https://skia-review.googlesource.com/7083
Commit-Queue: Jim Van Verth <jvanverth@google.com>
Reviewed-by: Jim Van Verth <jvanverth@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
10 files changed