[graphite] Add Buffer and MtlBuffer classes.

Bug: skia:12466
Change-Id: I3252f36c3f78642c55f127379ceba99b4845822e
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/458717
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
Reviewed-by: Jim Van Verth <jvanverth@google.com>
Commit-Queue: Greg Daniel <egdaniel@google.com>
7 files changed