All Apple GPUs support this, as does Mac GPU family 2. With this, we can
avoid expensive copies between buffers and textures allocated from the
same memory, and reduce memory usage to boot.
macOS is, unfortunately, constrained by the fact that `MTLHeap` objects
do not support any storage mode other than `MTLStorageModePrivate`--not
even `MTLStorageModeManaged`. This won't help for managed textures
there. Perhaps Apple will fix this in a later version.
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