[StThemeNode] Add helper method to get the paint allocation

StThemeNodes may have properties - namely shadows - which paint
outside an actor's allocation. This is not a problem unless drawing
is redirected to an offscreen buffer, in which case the actually
painted size is needed in advance when setting up the buffer.

Add a convenience method to calculate an allocation large enough to
paint the node.

https://bugzilla.gnome.org/show_bug.cgi?id=619025

src/st/st-theme-node.h

@@ -169,6 +169,10 @@ void st_theme_node_adjust_preferred_height (StThemeNode  *node,
 void st_theme_node_get_content_box         (StThemeNode        *node,
                                             const ClutterActorBox *actor_box,
                                             ClutterActorBox       *content_box);
+/* Helper for StThemeNodeTransition */
+void st_theme_node_get_paint_box           (StThemeNode           *node,
+                                            const ClutterActorBox *actor_box,
+                                            ClutterActorBox       *paint_box);
 
 gboolean st_theme_node_geometry_equal (StThemeNode *node,
                                        StThemeNode *other);