Ensure that blur result is identity if both sigmas are near-zero

Change-Id: I0c343167e63f147f21afee805ea934bd5f161024
Bug: chromium:1170700
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/360608
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
Commit-Queue: Jim Van Verth <jvanverth@google.com>
2 files changed