backend: Add pause/resume vfuncs to MetaBackend

For now we only pause/resume the rendering in the base class but in the
next few commits, we'll move udev from native to generic and need to
pause/resume it as well.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/4130>

src/backends/meta-backend-private.h

@@ -137,6 +137,10 @@ struct _MetaBackendClass
                                    MetaPointerConstraint *constraint);
 
   gboolean (* is_headless) (MetaBackend *backend);
+
+  void (* pause) (MetaBackend *backend);
+
+  void (* resume) (MetaBackend *backend);
 };
 
 void meta_backend_destroy (MetaBackend *backend);