[graphite] Add Combination to GraphicsPipelineDesc

Change-Id: I7fa9b4da48f993143ade3b8030e2c67831cf0ffd
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/475257
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Michael Ludwig <michaelludwig@google.com>
13 files changed