Graphite: add MtlGpu.

Bug: skia:12466
Change-Id: I51dc73a213e672578767b41c01c487c942465964
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/452722
Commit-Queue: Jim Van Verth <jvanverth@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
4 files changed