[graphite] Don't overload dstColor req flag with primitive color req

I may be missing something, but since kDestColor seems to be intended to
be used for setting up runtime blender arguments it seemed clearer to
make a separate requirement for blending against primitive colors.

Change-Id: Ibd3abdf3d3703794d674a3ad4f7cbc6948081966
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/651920
Reviewed-by: Nicolette Prevost <nicolettep@google.com>
2 files changed