On Wayland, opening and closing a `GdkDisplay` generates a coupe of
warnings at runtime:
```
GLib-GObject-WARNING **:
invalid cast from 'GdkWindowImplWayland' to 'GdkWindow'
GLib-GObject-WARNING **:
invalid cast from 'GdkWaylandWindow' to 'GdkWindowImplWayland'
```
This is from `gdk_window_impl_wayland_finalize()` which tries to cast
the given GObject to a `GdkWindow` while it's a `GdkWindowImplWayland`.
Use the correct type casting of objects to avoid the warnings.
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