Fill in missing GrMtlCaps functions and fix for 0 buffer alloc.

Bug: skia:
Change-Id: I4981e49dc45d607f3f6493da67844b6dfa74a551
Reviewed-on: https://skia-review.googlesource.com/c/189493
Commit-Queue: Greg Daniel <egdaniel@google.com>
Reviewed-by: Jim Van Verth <jvanverth@google.com>
3 files changed