Notice style transitions that don't change how StThemeNode paints

Add st_theme_node_paint_equal() and use that to do two things:

 1) Avoid animating transitions where nothing changes.
 2) Copy cached painting state from the old theme node to the new
    theme node.

https://bugzilla.gnome.org/show_bug.cgi?id=627083

src/st/st-theme-node.h

@@ -191,11 +191,15 @@ void st_theme_node_get_paint_box           (StThemeNode           *node,
 
 gboolean st_theme_node_geometry_equal (StThemeNode *node,
                                        StThemeNode *other);
+gboolean st_theme_node_paint_equal    (StThemeNode *node,
+                                       StThemeNode *other);
 
 void st_theme_node_paint (StThemeNode            *node,
                           const ClutterActorBox  *box,
                           guint8                  paint_opacity);
 
+void st_theme_node_copy_cached_paint_state (StThemeNode *node,
+                                            StThemeNode *other);
 
 G_END_DECLS