st-widget: Add method to clear background-image

For performance reasons, resources required to paint a widget are
aggressively cached; we know of at least one case where our caching
prevents updating the used background-image correctly, so add explicit
API to clear all associated cache data.

https://bugzilla.gnome.org/show_bug.cgi?id=679268

src/st/st-widget.h

@@ -142,6 +142,8 @@ ClutterActor *        st_widget_get_label_actor           (StWidget        *widg
 void                  st_widget_set_label_actor           (StWidget        *widget,
                                                            ClutterActor    *label);
 
+void                  st_widget_clear_background_image    (StWidget        *widget);
+
 /* Only to be used by sub-classes of StWidget */
 void                  st_widget_style_changed             (StWidget        *widget);
 StThemeNode *         st_widget_get_theme_node            (StWidget        *widget);