Draw Alpha8 + color filter correctly in software

This makes the behavior match our gpu backend.

BUG=android:31019366

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=3979

Change-Id: I941623d0bdf54f61aafb1383de46bbedceb7578d
Reviewed-on: https://skia-review.googlesource.com/3979
Reviewed-by: Mike Klein <mtklein@chromium.org>
Commit-Queue: Matt Sarett <msarett@google.com>
3 files changed