clutter/main: Use "is display server" state to decide a11y routing

We checked if we were using the usig the X11 backend to decide when to
deal with a11y event posting - in order to make the clutter code less
windowing system dependent, make this check a check whether we're a
display server or not, in contrast to a window/compositing manager
client. This is made into a vfunc ot ClutterBackendClass, implemented by
MetaClutterBackendNative and MetaClutterBackendX11.

https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1364

clutter/clutter/clutter-backend.c

@@ -1038,3 +1038,9 @@ clutter_backend_get_fallback_resource_scale (ClutterBackend *backend)
 {
   return backend->fallback_resource_scale;
 }
+
+gboolean
+clutter_backend_is_display_server (ClutterBackend *backend)
+{
+  return CLUTTER_BACKEND_GET_CLASS (backend)->is_display_server (backend);
+}