backend-native: Add non-functional 'headless' mode

Make it possible to pass --headless as a command line argument in order
to turn the native backend "headless". This currently doesn't do
anything, but the intention is that it should not use logind nor KMS,
and work completely headless with only virtual outputs.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1698>

src/backends/meta-backend.c

@@ -1458,14 +1458,20 @@ meta_backend_get_clutter_backend (MetaBackend *backend)
 }
 
 void
-meta_init_backend (GType backend_gtype)
+meta_init_backend (GType         backend_gtype,
+                   unsigned int  n_properties,
+                   const char   *names[],
+                   const GValue *values)
 {
   MetaBackend *backend;
   GError *error = NULL;
 
   /* meta_backend_init() above install the backend globally so
    * so meta_get_backend() works even during initialization. */
-  backend = g_object_new (backend_gtype, NULL);
+  backend = META_BACKEND (g_object_new_with_properties (backend_gtype,
+                                                        n_properties,
+                                                        names,
+                                                        values));
   if (!g_initable_init (G_INITABLE (backend), NULL, &error))
     {
       g_warning ("Failed to create backend: %s", error->message);