wayland/output: Flush clients after creating wl_output global

In order to give the clients the best chance to bind the wl_output
before we later remove it (for example on fast hot plugs or in the test
suite), flush the client sockets after creating the global.

https://bugzilla.gnome.org/show_bug.cgi?id=730551

src/wayland/meta-wayland-outputs.c

@@ -307,6 +307,8 @@ static MetaWaylandOutput *
 meta_wayland_output_new (MetaWaylandCompositor *compositor,
                          MetaLogicalMonitor    *logical_monitor)
 {
+  MetaWaylandCompositor *wayland_compositor =
+    meta_wayland_compositor_get_default ();
   MetaWaylandOutput *wayland_output;
 
   wayland_output = g_object_new (META_TYPE_WAYLAND_OUTPUT, NULL);
@@ -314,6 +316,7 @@ meta_wayland_output_new (MetaWaylandCompositor *compositor,
                                              &wl_output_interface,
                                              META_WL_OUTPUT_VERSION,
                                              wayland_output, bind_output);
+  meta_wayland_compositor_flush_clients (wayland_compositor);
   meta_wayland_output_set_logical_monitor (wayland_output, logical_monitor);
 
   return wayland_output;