Setting the extent on a tile source now only means that tiles
won't be drawn outside that extent. Now the only way to specify
the origin of the tile grid is to provide a custom tileGrid. By
default, the grid origin is the top left corner of the
projection's extent.
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.
