Guard some more Metal code to support older versions.

With these changes we can build across all versions of Metal, and for
the most part run on older OSes (further testing is necessary).

Bug: skia:9408
Change-Id: I00ee03b9a3a0e49483af4a610544b20d482a0cdd
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/245957
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Jim Van Verth <jvanverth@google.com>
15 files changed