Add some additional checks for shadow generation

Change-Id: I4dae4173ad879827e4e1fa3ad12aa0447d1df252
Reviewed-on: https://skia-review.googlesource.com/110360
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Jim Van Verth <jvanverth@google.com>
3 files changed