[graphite] Add 8-stop gradients

Bug: skia:13302
Change-Id: Id9927ffadfbbbc5578491284969736e3b5f60f26
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/541725
Reviewed-by: John Stiles <johnstiles@google.com>
Commit-Queue: James Godfrey-Kittle <jamesgk@google.com>
6 files changed