draw vertices: implement blending the tri-color shader

If there are colors, and the incoming paint has a shader,
then they need to be blended.

Bug=Skia:11822

Change-Id: Id5a1d7dffcaedb678214c694c9c87d0c71cdc991
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/434161
Reviewed-by: Mike Reed <reed@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Herb Derby <herb@google.com>
1 file changed