Update overlay example with center positioning

examples/overlay.js

@@ -1,5 +1,6 @@
 goog.require('ol.Map');
 goog.require('ol.Overlay');
+goog.require('ol.OverlayPositioning');
 goog.require('ol.RendererHints');
 goog.require('ol.View2D');
 goog.require('ol.coordinate');
@@ -22,10 +23,19 @@ var map = new ol.Map({
   })
 });
 
+var pos = ol.proj.transform([16.3725, 48.208889], 'EPSG:4326', 'EPSG:3857');
+
+// Vienna marker
+var marker = new ol.Overlay({
+  position: pos,
+  positioning: ol.OverlayPositioning.CENTER_CENTER,
+  element: document.getElementById('marker')
+});
+map.addOverlay(marker);
+
 // Vienna label
 var vienna = new ol.Overlay({
-  position: ol.proj.transform(
-      [16.3725, 48.208889], 'EPSG:4326', 'EPSG:3857'),
+  position: pos,
   element: document.getElementById('vienna')
 });
 map.addOverlay(vienna);