Extract code for safely creating SkMasks try 2

This is in preparation for adding a faster small radii blur.

Be sure to obey the original semantics about handling dst
when src.fImage is null.

Change-Id: Ic0ff0ac81494799e2d5037283904f1094309c622
Reviewed-on: https://skia-review.googlesource.com/70025
Commit-Queue: Florin Malita <fmalita@chromium.org>
Reviewed-by: Florin Malita <fmalita@chromium.org>
1 file changed