Fix transparent ambient shadows

Add a clamp value to indicate the percentage of the blurWidth to apply.

Change-Id: I962dc57ed786fa67ed1606b1148f1b2c568025e5
Reviewed-on: https://skia-review.googlesource.com/16424
Commit-Queue: Jim Van Verth <jvanverth@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
6 files changed