gdkdisplay-wayland: Init selection before roundtrips

Some of the listeners connected during the initial roundtrip assume a
non-NULL selection, when they finally invoke functions such as
gdk_wayland_selection_ensure_offer(),
gdk_wayland_selection_ensure_primary_offer() or
gdk_wayland_selection_set_offer(). The GdkWaylandSelection should
therefore be initialized before the second series of roundtrips, as the
compositor may have already sent a selection by then.

gdk/wayland/gdkdisplay-wayland.c

@@ -640,6 +640,7 @@ _gdk_wayland_display_open (const gchar *display_name)
     }
 
   process_on_globals_closures (display_wayland);
+  display_wayland->selection = gdk_wayland_selection_new ();
 
   /* Wait for initializing to complete. This means waiting for all
    * asynchrounous roundtrips that were triggered during initial roundtrip. */
@@ -684,8 +685,6 @@ _gdk_wayland_display_open (const gchar *display_name)
       return NULL;
     }
 
-  display_wayland->selection = gdk_wayland_selection_new ();
-
   g_signal_emit_by_name (display, "opened");
 
   return display;