[graphite] Expand GradientData to include everything needed for uniform generation

Bug: skia:12701
Change-Id: Ib081e07731d30d5da19fef9191958ee3ee3a23a5
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/502777
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
3 files changed