add first draft of onScreen() function to marker

git-svn-id: http://svn.openlayers.org/trunk/openlayers@532 dc9f47b5-9b13-0410-9fdd-eb0c1a62fdaf

lib/OpenLayers/Marker.js

@@ -59,6 +59,24 @@ OpenLayers.Marker.prototype = {
         }            
     },
 
+    /**
+     * @returns Whether or not the marker is currently visible on screen.
+     * @type Boolean
+     */
+    onScreen:function() {
+
+        var screenBounds = OpenLayers.Bounds.fromSize(this.map.getSize());  
+
+        var iconTop = parseInt(this.icon.div.style.top);
+        var iconLeft = parseInt(this.icon.div.style.left);
+        var iconBottom = iconTop + parseInt(this.icon.div.style.height);
+        var iconRight = iconLeft + parseInt(this.icon.div.style.width);
+        var iconBounds = 
+           new OpenLayers.Bounds(iconLeft, iconBottom, iconRight, iconTop);
+
+        return screenBounds.containsBounds(iconBounds, true);
+    },
+    
     /**
      * @param {float} inflate
      */