Revamp pixelformat table for Metal

Bug: skia:6718
Change-Id: I0798d843647b4de0c655f3f01dc2ae76899777fe
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/227638
Commit-Queue: Robert Phillips <robertphillips@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>
3 files changed