[graphite] New Combination API

Bug: skia:13430
Change-Id: I816c57d0ed8694d21911c449d67c035580dd63d1
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/548776
Reviewed-by: Herb Derby <herb@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
7 files changed