2D/Columbus-view artifact with extent geometries at the pole

From https://groups.google.com/forum/#!forum/cesium-dev

In the [Geometry and Appearances Sandcastle example](http://cesium.agi.com/Cesium/Apps/Sandcastle/index.html?src=Geometry%20and%20Appearances.html&label=Showcases), replace Line 119 with

``` javascript
extent = Cesium.Extent.fromDegrees(-180.0, 45.0, -160.0, 90.0);
```

Given that `90.0` is at the pole, we get artifacts in 2D and columbus view.

3D:


2D:


Using `89.99` instead of `90.0` does not cause artifacts.

Perhaps the geometry is optimizing for this in 3D and not introducing a northern row. If that's the case, we can pass an `allow3DOnly` option that defaults to `false`.