Camera.computeViewRectangle doesn't work in 2D or CV

Reported on the forum: https://groups.google.com/forum/?hl=en#!topic/cesium-dev/Ildibeod608

``` javascript
var viewer = new Cesium.Viewer('cesiumContainer', {
sceneMode: Cesium.SceneMode.SCENE2D
});

viewer.scene.camera.flyTo({
destination: Cesium.Cartesian3.fromDegrees(-90, 43, 50000),
duration: 0,
});

viewer.camera.moveEnd.addEventListener(logViewRectangle);

function logViewRectangle() {
console.log('camera.computeViewRectangle() is ' + JSON.stringify(viewer.scene.camera.computeViewRectangle()));
}
```

Reproduce the provided Cesium Viewer example in SCENE2D and inspect Camera.computeViewRectangle() after camera.moveEnd. Compare its behavior in 2D and Columbus View, then make the result represent the visible camera rectangle in both modes and verify the reported example no longer returns an unusable result.

Written by the indexing model from the issue text.