cursor-tracker: Don't require a screen

This allows us to do initialization earlier and not have to poke into
Wayland internals from the cursor tracker.

src/core/screen.c

@@ -667,6 +667,9 @@ meta_screen_new (MetaDisplay *display,
       return NULL;
     }
 
+  /* Select for cursor changes so the cursor tracker is up to date. */
+  XFixesSelectCursorInput (xdisplay, xroot, XFixesDisplayCursorNotifyMask);
+
   screen = g_object_new (META_TYPE_SCREEN, NULL);
   screen->closing = 0;