[graphite] Add index buffer support and instanced draws

Bug: skia:12466
Change-Id: I12c64396791d8e0b8891b82927b1c8811a6e164f
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/464385
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Jim Van Verth <jvanverth@google.com>
7 files changed