Convert ol.render.Event#get* methods into properties

examples/export-map.js

@@ -24,7 +24,7 @@ var exportPNGElement = document.getElementById('export-png');
 if ('download' in exportPNGElement) {
   exportPNGElement.addEventListener('click', function(e) {
     map.once('postcompose', function(event) {
-      var canvas = event.getContext().canvas;
+      var canvas = event.context.canvas;
       exportPNGElement.href = canvas.toDataURL('image/png');
     });
     map.render();