This makes sure that hidden widgets always have priv->alloc_needed set
on them.
The constructor sets that flag, so we want to have it back when we
revert to this state.
This fixes GtkWindow skipping a size_allocate() when reshowing a
previously hidden window and thereby not updating its allocation and
clip. And that in turn would lead to draws not happening and us beig
left with a black window.
if (parent)
gtk_widget_queue_resize (parent);
+ gtk_widget_queue_allocate (widget);
+
gtk_widget_pop_verify_invariants (widget);
g_object_unref (widget);
}