toytoolkit: Only require xdg_shell if the window is not custom

Signed-off-by: Jason Ekstrand <jason@jlekstrand.net>

clients/window.c

@@ -4359,7 +4359,7 @@ window_create_internal(struct display *display, int custom)
 	surface = surface_create(window);
 	window->main_surface = surface;
 
-	fail_on_null(display->xdg_shell);
+	assert(custom || display->xdg_shell);
 
 	window->custom = custom;
 	window->preferred_format = WINDOW_PREFERRED_FORMAT_NONE;