Web Mercator Weirdness

I tried enabling WebMercator mode like this:

```
var viewer = new Cesium.Viewer('cesiumContainer', {
mapProjection: new Cesium.WebMercatorProjection(Cesium.Ellipsoid.WGS84)
});
//viewer.extend(Cesium.viewerCesiumInspectorMixin);
viewer.scene.morphTo2D(0);
```

But the world went awash in wacky weirdness:


Is this a bug or am I just doing it wrong? If it's not a bug then we might need clearer docs on how to do this.

/cc @kring @abwood

Reproduce the issue with Cesium.Viewer, WebMercatorProjection, and scene.morphTo2D using the snippet in the report. Trace how mapProjection is handled during the 2D morph and determine whether the rendering is incorrect or the usage needs clarification; done means the behavior is fixed or the supported setup is documented.

Written by the indexing model from the issue text.