gdk/wayland: cope with the compositor implementing an older protocol than us

If we bind to a global with an higher version than implemented, or
we make requests that appeared in a later version, we would get
fatal wayland errors.

https://bugzilla.gnome.org/show_bug.cgi?id=704104

gdk/wayland/gdkdisplay-wayland.h

@@ -67,6 +67,7 @@ struct _GdkWaylandDisplay
   GSource *event_source;
 
   int init_ref_count;
+  int compositor_version;
 
   struct xkb_context *xkb_context;
 };