SceneTransforms.wgs84ToWindowCoordinates returns undefined for certain camera angles

Reported on the forum: https://groups.google.com/forum/?hl=en#!topic/cesium-dev/ALIPbA_LSqU

``` javascript
var viewer = new Cesium.Viewer('cesiumContainer');
viewer.camera.lookAt(Cesium.Cartesian3.fromDegrees(20.5, 54.7),
new Cesium.Cartesian3(35000.0, -200000.0, 100000.0));
viewer.camera.lookAtTransform(Cesium.Matrix4.IDENTITY);
var scene = viewer.scene;
var ellipsoid = scene.globe.ellipsoid;
var position = Cesium.Cartesian3.fromDegrees(0.0, 0.0);
var handler = new Cesium.ScreenSpaceEventHandler(scene.canvas);
handler.setInputAction(function(movement) {
console.log(Cesium.SceneTransforms.wgs84ToWindowCoordinates(scene, position));
}, Cesium.ScreenSpaceEventType.LEFT_CLICK);
```

Start at SceneTransforms.wgs84ToWindowCoordinates and reproduce the supplied Viewer, camera-angle, and click-handler example. Trace the coordinate conversion for the reported camera orientation and compare it with a working orientation. Done means the conversion has defined, usable behavior for the reported case, with a regression test if the surrounding test structure supports one.

Written by the indexing model from the issue text.