Metal: add loadMSAAFromResolve support.

Also fixes some minor warnings.

Bug: skia:12086
Change-Id: Ia476a7a196b490022978761e92f935a6d668136a
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/441797
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Jim Van Verth <jvanverth@google.com>
7 files changed