Add compressed backend textures to Metal

Bug: skia:9680
Change-Id: I7a3759d60549fd5f9051f57df0f4918c307ea3c8
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/265402
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
3 files changed