GtkThemingEngine: Add vmethod to render radio buttons.

gtk/gtkthemingengine.h

@@ -55,6 +55,12 @@ struct GtkThemingEngineClass
                          gdouble           y,
                          gdouble           width,
                          gdouble           height);
+  void (* render_option) (GtkThemingEngine *engine,
+                          cairo_t          *cr,
+                          gdouble           x,
+                          gdouble           y,
+                          gdouble           width,
+                          gdouble           height);
 };
 
 GType gtk_theming_engine_get_type (void) G_GNUC_CONST;