Add mipmap generation for Metal

Bug: skia:8243
Change-Id: Id71073dffe6e2b42d099f0c92059b53c48b46c4f
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/206717
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Jim Van Verth <jvanverth@google.com>
4 files changed