Update window rect when creating toplevel

This one is a bit wild. Basically the fact that multiwin input for child
toplevels was basically a lot of luck.

In short, the window node still believed it had a position and
transformed all input, confusing the hell out of it. This does not
happen on most "modern" compositors as they implement fractional
scaling, which triggers a rect update, but Weston did not include that
protocol and never sent the update.

Well, that's what a reference compositor is for, right? :P

platform/linuxbsd/wayland/display_server_wayland.cpp

@@ -611,6 +611,12 @@ void DisplayServerWayland::show_window(WindowID p_window_id) {
 			if (wd.parent_id != INVALID_WINDOW_ID) {
 				wayland_thread.window_set_parent(wd.id, wd.parent_id);
 			}
+
+			// The window node believes it can have a position. It can't. Let's tell it
+			// the news by sending it the actual rect.
+			if (wd.rect_changed_callback.is_valid()) {
+				wd.rect_changed_callback.call(wd.rect);
+			}
 		} else {
 			DEBUG_LOG_WAYLAND("!!!!! Making popup !!!!!");