Access GTK window before it's mapped

[andrewbaxter]

Is your feature request related to a problem? Please describe.
I'm trying to do wl-layer-shell stuff on the window the webview is in. Per tauri-apps/wry#1253 I am able to access the gtk window, but it's already mapped at that point it seems (is this due to the redraw call?):

custom_shell_surface_init: assertion '!gtk_widget_get_mapped (GTK_WIDGET (gtk_window))' failed
GtkWindow is not a layer surface. Make sure you called gtk_layer_init_for_window ()
GtkWindow is not a layer surface. Make sure you called gtk_layer_init_for_window ()
GtkWindow is not a layer surface. Make sure you called gtk_layer_init_for_window ()
GtkWindow is not a layer surface. Make sure you called gtk_layer_init_for_window ()
GtkWindow is not a layer surface. Make sure you called gtk_layer_init_for_window ()
GtkWindow is not a layer surface. Make sure you called gtk_layer_init_for_window ()
GtkWindow is not a layer surface. Make sure you called gtk_layer_init_for_window ()
GtkWindow is not a layer surface. Make sure you called gtk_layer_init_for_window ()

Describe the solution you'd like
I'm not sure the best way to do this... maybe in the long run a wl_layer_shell abstraction that could handle other platforms as well would be good.

Either being able to prevent the initial draw/put it off until later (maybe until the event loop starts?) or provide a callback to do stuff with the window earlier on would probably work.

Describe alternatives you've considered
A way to "bring your own window" might be a more flexible solution, for when cross-platform support isn't needed.

Additional context