wayland: Ensure we queue a "calc showing" operation after we get a buffer

This will ensure the window is made visible, now that we're going to ignore
all previous petitions until we get a buffer.

https://bugzilla.gnome.org/show_bug.cgi?id=750552

src/wayland/meta-wayland-surface.c

@@ -486,6 +486,9 @@ apply_pending_state (MetaWaylandSurface      *surface,
 
   if (pending->newly_attached)
     {
+      if (!surface->buffer && surface->window)
+        meta_window_queue (surface->window, META_QUEUE_CALC_SHOWING);
+
       surface_set_buffer (surface, pending->buffer);
 
       if (pending->buffer)