Metal: disable use of discardable MSAA on non-Apple silicon.

Resolves appear to be very slow on non-tiled desktop GPUs.

Bug: skia:12447
Change-Id: Ic6fcad667c63cac764fe0ee3d3797159df45d27a
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/449176
Auto-Submit: Jim Van Verth <jvanverth@google.com>
Commit-Queue: Greg Daniel <egdaniel@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>
1 file changed