give the element a backgroundGraphic so we know that we need to remove the background element

git-svn-id: http://svn.openlayers.org/trunk/openlayers@7997 dc9f47b5-9b13-0410-9fdd-eb0c1a62fdaf

tests/Renderer/Elements.html

@@ -428,7 +428,8 @@
                 'removeChild': function(elem) {
                     gElemRemoved = elem;
                 }
-            }
+            },
+            '_style' : {backgroundGraphic: "foo"}
         };
         gBackElement = {
             'parentNode': {