Snap y when updating cubic

If we don't snap, we could have end up with many tiny line segments
wihtin a single row of pixels. That combined with our partial alpha
rounding code (https://codereview.chromium.org/2483523002/) could
accumlate so much error that violates our alpha <= 256 assert.

BUG=skia:

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=4724

Change-Id: I58b02cffc8a4dc9f541e194d61a2f6fef7b85c97
Reviewed-on: https://skia-review.googlesource.com/4724
Reviewed-by: Cary Clark <caryclark@google.com>
Commit-Queue: Yuqian Li <liyuqian@google.com>
1 file changed