scale: Fix sporadic criticals
gtk_css_node_update_layout_attributes can cause us to free priv->layout, and then bad things happen. Therefore, we must call that function on a new layout before setting priv->layout.
gtk_css_node_update_layout_attributes can cause us to free priv->layout, and then bad things happen. Therefore, we must call that function on a new layout before setting priv->layout.