libweston-desktop/xwayland: XWAYLAND surfaces are never 'added'

Add documentation (asserts) that show that windows of types XWAYLAND are
never registered with the shell.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Quentin Glidic <sardemff7+git@sardemff7.net>

libweston-desktop/xwayland.c

@@ -83,6 +83,8 @@ weston_desktop_xwayland_surface_change_state(struct weston_desktop_xwayland_surf
 
 	if (surface->state != state) {
 		if (surface->state == XWAYLAND) {
+			assert(!surface->added);
+
 			weston_desktop_surface_unlink_view(surface->view);
 			weston_view_destroy(surface->view);
 			surface->view = NULL;
@@ -100,6 +102,8 @@ weston_desktop_xwayland_surface_change_state(struct weston_desktop_xwayland_surf
 		}
 
 		if (state == XWAYLAND) {
+			assert(!surface->added);
+
 			surface->view =
 				weston_desktop_surface_create_view(surface->surface);
 			weston_layer_entry_insert(&surface->xwayland->layer.view_list,