[graphite] Initial shader combination/precompile API

Bug: skia:12466
Change-Id: I49e1ec8308a0cde2b5d43901281d60a6f70be5c4
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/457119
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
3 files changed