meta/boxes: typedef cairo_rectangle_int_t MetaRectangle

They are already effectively interchangeable so this should reduce
pointless casts.

Just like in GDK though, we need to keep the old definition for
instrospection to be able to include the struct's fields.

src/core/boxes-private.h

@@ -215,6 +215,4 @@ GList* meta_rectangle_find_nonintersected_monitor_edges (
                                            const GList         *monitor_rects,
                                            const GSList        *all_struts);
 
-cairo_rectangle_int_t meta_rectangle_to_cairo_rectangle (MetaRectangle *rect);
-
 #endif /* META_BOXES_PRIVATE_H */