Overriding the `layer` on `NSView` makes the view "layer-hosting", see
[wantsLayer], which disables drawing functionality on the view like
`drawRect:`/`updateLayer`.

This prevents crates like Winit from providing a robust rendering
callback that integrates well with the rest of the system.

Instead, if the layer is not CAMetalLayer, we create a new sublayer, and
render to that instead.

[wantsLayer]: https://developer.apple.com/documentation/appkit/nsview/1483695-wantslayer?language=objc

We do not need to use it, and the layer itself is already retained, so
it won't be de-allocated from under our feet.