Sometimes, the `CAMetalLayer` backing a view can be replaced--for
example, when the window is moved to another screen, or the style bits
on the window are changed. In that case, we must allow the client the
opportunity to recreate the surface and swapchain.
Layer-backed views always set themselves as the layer's delegate; we use
this fact to Key-Value Observe the view's `layer` property.
Other alternatives considered:
* Registering for `NSViewGlobalFrameDidChange` notifications. Aside from
only working on macOS, this doesn't actually catch every case where we
want to report a lost surface. I'm not even sure it works at all for
Metal.
* Holding a reference to the view, and checking when its layer property
has changed.
* Holding a weak reference to the layer; that way, the reference will
become `nil` when the layer is replaced. But this requires ARC.
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