core: enable XInput2 by default

Enable XI2 support in both Clutter and GDK.

https://bugzilla.gnome.org/show_bug.cgi?id=688779

src/ui/ui.c

@@ -60,14 +60,6 @@ struct _MetaUI
 void
 meta_ui_init (void)
 {
-  /* As of 2.91.7, Gdk uses XI2 by default, which conflicts with the
-   * direct X calls we use - in particular, events caused by calls to
-   * XGrabPointer/XGrabKeyboard are no longer understood by GDK, while
-   * GDK will no longer generate the core XEvents we process.
-   * So at least for now, enforce the previous behavior.
-   */
-  gdk_disable_multidevice ();
-
   if (!gtk_init_check (NULL, NULL))
     meta_fatal ("Unable to open X display %s\n", XDisplayName (NULL));