Polylines with height don't show up in rotatable 2D mode

This polyline doesn't show up on the globe. If you remove the height, or change to regular 2D mode, it shows up fine.

``` javascript
var viewer = new Cesium.Viewer('cesiumContainer', {
sceneMode: Cesium.SceneMode.SCENE2D,
mapMode2D : Cesium.MapMode2D.ROTATE
});

var orangeOutlined = viewer.entities.add({
polyline : {
positions : Cesium.Cartesian3.fromDegreesArrayHeights([-75, 39, 250000,
-125, 39, 250000]),
width : 5
}
});

viewer.zoomTo(viewer.entities);
```

Start with the provided Cesium Viewer reproduction using SCENE2D and MapMode2D.ROTATE, then trace how height-bearing entity polylines are rendered in that mode. Done means the sample polyline appears in rotatable 2D mode while retaining its height.

Written by the indexing model from the issue text.