scene: add knob to turn off Xwayland surface restacking

This is useful in these cases:

- The same surface is added to two different scene-graphs. wlroots
  can't figure out on its own which scene-graph should drive the
  Xwayland stacking.
- A compositor uses multiple Xwayland servers (e.g. one per app).
  wlroots will try to restack surfaces from different Xwayland
  instances and this will not go well.

include/wlr/types/wlr_scene.h

@@ -105,6 +105,8 @@ struct wlr_scene {
 	struct wlr_gamma_control_manager_v1 *gamma_control_manager_v1;
 	struct wlr_color_manager_v1 *color_manager_v1;
 
+	bool restack_xwayland_surfaces;
+
 	struct {
 		struct wl_listener linux_dmabuf_v1_destroy;
 		struct wl_listener gamma_control_manager_v1_destroy;