extract common code from blur

This CL extracts the very fiddly edge case code that should be common
to both blurs. This is a single step in the progression to share
even more code.

Change-Id: I9d22cb8ae44e7ff2cb49196a3c0b464e48c21cdc
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/441062
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Herb Derby <herb@google.com>
1 file changed