Reland "Metal: add loadMSAAFromResolve support."

This is a reland of 9f82158f17ee638e11cf68cbb07d0cec1a4c847c

Original change's description:
> 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>

Bug: skia:12086
Change-Id: I1d9dd36b3f8f19a0ab4a98ccaab18146566c1885
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/446077
Commit-Queue: Jim Van Verth <jvanverth@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>
7 files changed