Use AdditiveBlitter for partial rows

Previously, we forgot to use AdditiveBlitter in two places where partial
rows are blitterred. That causes SkAAClip to complain as in skia:6003.

BUG=skia:6003

Change-Id: I4f4a896072448bdb3f287a2eb61cb64b1256ea78
Reviewed-on: https://skia-review.googlesource.com/5273
Reviewed-by: Cary Clark <caryclark@google.com>
Commit-Queue: Yuqian Li <liyuqian@google.com>
2 files changed