Fix fast blur for shadow rrects.

Fix fast blur for shadow rrects.

* Handles case where blur is greater than
  corner radius.
* Speeds up shader by avoiding a divide.
* Tweaks shadow sample to be more MD-ish.

Bug: skia:
Change-Id: I2990357b39784c22f02a8e51872391fd68e8226a
Reviewed-on: https://skia-review.googlesource.com/14141
Commit-Queue: Jim Van Verth <jvanverth@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>