Initialize dst image memory for small sigma

The GPU and CPU share the same code for layout; the layout is too large for CPU. So,
the CPU code must clear some of the destination even if there would be no work to do.

Change-Id: I29a04217b620b033a01db53487dc64e377661436
BUG=chromium:789812
Reviewed-on: https://skia-review.googlesource.com/79401
Commit-Queue: Herb Derby <herb@google.com>
Reviewed-by: Florin Malita <fmalita@chromium.org>
1 file changed