x11: Introduce a meta-x11-types header

Used to define x11 only types. Future commits will introduce
a way for compositors to detect if they can use x11 features or not

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3776>

src/x11/meta-x11-display.c

@@ -2572,3 +2572,14 @@ meta_x11_display_lookup_xwindow (MetaX11Display *x11_display,
 
   return None;
 }
+
+/**
+ * meta_display_get_x11_display: (skip)
+ * @display: a #MetaDisplay
+ *
+ */
+MetaX11Display *
+meta_display_get_x11_display (MetaDisplay *display)
+{
+  return display->x11_display;
+}