xwayland/xwm: introduce wlr_xwayland_surface_try_from_wlr_surface()

This new function replaces wlr_surface_is_xwayland_surface() and
wlr_xwayland_surface_from_wlr_surface().

include/wlr/xwayland/xwayland.h

@@ -231,20 +231,13 @@ void wlr_xwayland_surface_set_fullscreen(struct wlr_xwayland_surface *surface,
 void wlr_xwayland_set_seat(struct wlr_xwayland *xwayland,
 	struct wlr_seat *seat);
 
-/**
- * Check whether a surface is an Xwayland surface.
- *
- * As an edge case, if the surface has been created by Xwayland but has no X11
- * window associated, false is returned.
- */
-bool wlr_surface_is_xwayland_surface(struct wlr_surface *surface);
-
 /**
  * Get a struct wlr_xwayland_surface from a struct wlr_surface.
  *
- * This function asserts that the surface is an Xwayland surface.
+ * If the surface hasn't been created by Xwayland or has no X11 window
+ * associated, NULL is returned.
  */
-struct wlr_xwayland_surface *wlr_xwayland_surface_from_wlr_surface(
+struct wlr_xwayland_surface *wlr_xwayland_surface_try_from_wlr_surface(
 	struct wlr_surface *surface);
 
 void wlr_xwayland_surface_ping(struct wlr_xwayland_surface *surface);

xwayland/xwm.c

@@ -101,14 +101,12 @@ struct pending_startup_id {
 
 static const struct wlr_addon_interface surface_addon_impl;
 
-bool wlr_surface_is_xwayland_surface(struct wlr_surface *surface) {
-	return wlr_addon_find(&surface->addons, NULL, &surface_addon_impl) != NULL;
-}
-
-struct wlr_xwayland_surface *wlr_xwayland_surface_from_wlr_surface(
+struct wlr_xwayland_surface *wlr_xwayland_surface_try_from_wlr_surface(
 		struct wlr_surface *surface) {
 	struct wlr_addon *addon = wlr_addon_find(&surface->addons, NULL, &surface_addon_impl);
-	assert(addon != NULL);
+	if (addon == NULL) {
+		return NULL;
+	}
 	struct wlr_xwayland_surface *xsurface = wl_container_of(addon, xsurface, surface_addon);
 	return xsurface;
 }