Fix mask overflow caused by edge drift

BUG=skia:

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=4797

Change-Id: Ica1568b67c1e1ce4aae2bdaba2c5b1f2155d1382
Reviewed-on: https://skia-review.googlesource.com/4797
Reviewed-by: Cary Clark <caryclark@google.com>
3 files changed