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>
diff --git a/gn/gm.gni b/gn/gm.gni
index 6d68f12..6a0b89f 100644
--- a/gn/gm.gni
+++ b/gn/gm.gni
@@ -59,6 +59,7 @@
   "$_gm/color4f.cpp",
   "$_gm/colorcube.cpp",
   "$_gm/coloremoji.cpp",
+  "$_gm/colorfilteralpha8.cpp",
   "$_gm/colorfilterimagefilter.cpp",
   "$_gm/colorfilters.cpp",
   "$_gm/colormatrix.cpp",