Fix GPU-only snapping bug in mask blur rendering

Fix GPU-only snapping bug in mask blur rendering

The existing mask effect code in Ganesh is subject to snapping issues (when the created mask is redrawn). This artifact can be seen by rendering the original geometry (w/o blurs) and comparing that result to a rendering of the unblurred masks. W/o this patch the results do not match up (they are arbitrarily shifted by a pixel).

This patch will require rebaselining and suppressions.

Chromium layout tests suppressions are here:
https://codereview.chromium.org/1342683003/ (Add layout test suppressions for upcoming Skia roll)

Review URL: https://codereview.chromium.org/1338183002