Add ShadowFlag to use only blurs for concave shapes.

The geometric method for concave shadows seems to be failing in certain
cases. See: https://github.com/flutter/flutter/issues/84262. I've never
been satisfied with this solution and I've been thinking of removing it. This flag allows for Flutter to disable it for the time being until I
can determine if anyone else is using it.

Change-Id: Ia0a3f57002d94928f2baa655c88dc4d10b9edef8
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/533881
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Jim Van Verth <jvanverth@google.com>
3 files changed