Add compressed backend textures to Metal (take 2)

Bug: skia:9680
Change-Id: I88035added2fd1721ef98048bd3344ab90e3da5c
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/266199
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
3 files changed