[css2] Added bbox

--HG--
extra : convert_revision : svn%3A73dc7c4b-06e6-40f3-b4f7-9ed1dbc14bfc/trunk%401159

Author: ianbjacobs

css2/properties/descriptors.db

@@ -42,4 +42,4 @@ undefined ;; visual
 /* centerline;; <number>;; undefined ;; visual*/
 /* mathline;; <number>;; undefined ;; visual*/
 /* topline;; <number>;; undefined ;; visual*/
-
+/* bbox;; <number>, <number>, <number>, <number>;; undefined;; visual */