Change getUrlAsync parameters.

Instead of giving the function a bound, a scope, a property to update
and a callback, only give the bound and a callback.

When the url is retrieved by getUrlAsync, simply call the callback
with the url as argument and let the caller manage this.

tests/Tile/Image.html

@@ -85,9 +85,8 @@
         var layer = new OpenLayers.Layer.WMS(
             "Name",
             "http://labs.metacarta.com/TESTURL?",
-            {layers: 'basic'}, {async: true, getURLasync: function(bounds, scope, url, callback) {
-                scope.url = this.getURL(bounds);
-                callback.call(scope);
+            {layers: 'basic'}, {async: true, getURLasync: function(bounds, callback) {
+                callback(this.getURL(bounds));
             }}
         ); 
         map.addLayer(layer);