Adapt GrRectBlur for cases with rect W/H less than six sigma.

Add GM that generates reference blur rect images and compares against
actual.

Change-Id: If0ce291e211fefe96af8afdf0a60636b5f40ef47
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/239757
Commit-Queue: Brian Salomon <bsalomon@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
10 files changed