add onScreen() function to OpenLayers.Feature, and add tests to prove it.

git-svn-id: http://svn.openlayers.org/trunk/openlayers@1510 dc9f47b5-9b13-0410-9fdd-eb0c1a62fdaf

lib/OpenLayers/Feature.js

@@ -72,6 +72,21 @@ OpenLayers.Feature.prototype= {
         }
     },
     
+    /**
+     * @returns Whether or not the feature is currently visible on screen
+     *           (based on its 'lonlat' property)
+     * @type Boolean
+     */
+    onScreen:function() {
+        
+        var onScreen = false;
+        if ((this.layer != null) && (this.layer.map != null)) {
+            var screenBounds = this.layer.map.getExtent();
+            onScreen = screenBounds.containsLonLat(this.lonlat);
+        }    
+        return onScreen;
+    },
+    
 
     /**
      * @returns A Marker Object created from the 'lonlat' and 'icon' properties