We only want the window server to use the high-performance GPU if we
will use it to present to the display. If we won't use it to present, we
can save some battery life by not using the display. I had hoped this
would help window server stability in case something goes horribly
wrong while using the GPU, but my experience has sadly not borne this
out.
My testing shows that the device returned by
`MTLCreateSystemDefaultDevice()` is exactly equal (i.e. has the same
pointer value) to one of the devices returned by `MTLCopyAllDevices()`,
so we should see no problems from doing this at swapchain create time
instead of device create time.
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