[Graphite] Add general support for render steps to emit a primitive color

Also a fix small comment in SubRunData.

Bug: b/238758223
Change-Id: Idf0943992aeb60ffd96c62a579c3f910ac0c7fc8
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/569328
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
Commit-Queue: Nicolette Prevost <nicolettep@google.com>
3 files changed