Add MtlTexture class.

Bug: skia:12466
Change-Id: I6207af8bb34ccab28265ced733e1930f42e2db8b
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/454776
Reviewed-by: Robert Phillips <robertphillips@google.com>
Reviewed-by: Jim Van Verth <jvanverth@google.com>
Commit-Queue: Greg Daniel <egdaniel@google.com>
3 files changed