Point clouds broken in Columbus View


I'm not sure if this is a CV specific bug, a Point Clouds specific bug, or a general 3D Tiles bug, but there is something very wrong with Point Clouds in Columbus View, here's a complete snippet (It's just the stock sandcastle with CV mode enabled):

```js
//Point Cloud by Prof. Peter Allen, Columbia University Robotics Lab. Scanning by Alejandro Troccoli and Matei Ciocarlie.
var viewer = new Cesium.Viewer('cesiumContainer', {sceneMode: Cesium.SceneMode.COLUMBUS_VIEW});

var tileset = viewer.scene.primitives.add(new Cesium.Cesium3DTileset({
url : 'https://beta.cesium.com/api/assets/1460?access_token=eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJqdGkiOiIyMzk2YzJiOS1jZGFmLTRlZmYtYmQ4MS00NTA3NjEwMzViZTkiLCJpZCI6NDQsImFzc2V0cyI6WzE0NjBdLCJpYXQiOjE0OTkyNjQ3NTV9.oWjvN52CRQ-dk3xtvD4e8ZnOHZhoWSpJLlw115mbQJM'
}));

tileset.readyPromise.then(function() {
var boundingSphere = tileset.boundingSphere;
viewer.camera.viewBoundingSphere(boundingSphere, new Cesium.HeadingPitchRange(0.0, -0.5, boundingSphere.radius));
viewer.camera.lookAtTransform(Cesium.Matrix4.IDENTITY);
}).otherwise(function(error) {
throw(error);
});
```

CC @lilleyse

Reproduce the provided Cesium.Viewer and Cesium3DTileset snippet with SceneMode.COLUMBUS_VIEW and asset 1460 in a stock Sandcastle example. Inspect the point-cloud and 3D Tiles rendering path to isolate the Columbus View failure; done means the point cloud displays correctly in that scene mode without breaking the existing example.

Written by the indexing model from the issue text.