[graphite] Deduplicate the paint color uniform

Bug: b/251493570
Change-Id: Ic47e25183fef84f239abc1e3b7914ab0bf8e9c95
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/757318
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
17 files changed