Add an average mode for sigma < 2

The original code had an optimization when sigma < 2 that averaged
pixels instead of Gaussian bluring them. This CL adds that
behavior back to the new implementat.

BUG=chromium:745290
Change-Id: I35b7de2138a859d546439cc2053b4b599a94ebe1
Reviewed-on: https://skia-review.googlesource.com/34180
Commit-Queue: Herb Derby <herb@google.com>
Reviewed-by: Mike Klein <mtklein@chromium.org>
2 files changed