Add GrMtlCommandBuffer

Used to encapsulate encoder creation. Bundles blitencoder
commands into a single encoder.

Bug: skia:8243
Change-Id: I53a4303678bb4f4e6667a3655cfe414b2e50615d
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/209435
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Jim Van Verth <jvanverth@google.com>
8 files changed