a swipe control for openalyers2 | 卷帘控件
var map = new new OpenLayers.Map(mapdiv);
var swipe = new OpenLayers.Control.Swipe({map: map}); //实例控件
map.addControls([swipe]); //添加控件
swipe.activate(layer); //激活控件,传layer指定上层被clip图层
swipe.deactivate(); //控件失效