Metal: Implement ASTC LDR support Bug: angleproject:2634 Change-Id: If7ea09de384c8bc8727523f20fe17641127fcf9f Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2411982 Commit-Queue: Jonah Ryan-Davis <jonahr@google.com> Reviewed-by: Le Hoang Quyen <le.hoang.q@gmail.com> Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>