add stage for gaussian alpha to rgba for shadows

speeds up GM:shadow_utils 20%

Bug: skia:
Change-Id: If52dd5e2c76ace82d06351af1419e0663a3a634f
Reviewed-on: https://skia-review.googlesource.com/17844
Commit-Queue: Mike Klein <mtklein@chromium.org>
Reviewed-by: Mike Klein <mtklein@chromium.org>
5 files changed