Use fast path for circular shadows.

Also cleans up some of the MaskFilter code to make it
more correct.

BUG=skia:6119

Change-Id: I93016bcdd9c55fcb2d1dc8776428a72eb563d67a
Reviewed-on: https://skia-review.googlesource.com/9116
Commit-Queue: Jim Van Verth <jvanverth@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
4 files changed