Cross origin handling to make example work in Firefox

The WMS used in the example does not send CORS headers (I think
not many WMS services do). It seems that the WebGL renderer
needs script access to the image data, so the example only works
with the DOM renderer in Firefox.

src/ol/source/tilesource.js

@@ -14,7 +14,7 @@ goog.require('ol.tilegrid.TileGrid');
 
 /**
  * @typedef {{attributions: (Array.<ol.Attribution>|undefined),
- *            crossOrigin: (string|undefined),
+ *            crossOrigin: (null|string|undefined),
  *            extent: (ol.Extent|undefined),
  *            projection: (ol.Projection|undefined),
  *            tileGrid: (ol.tilegrid.TileGrid|undefined),