Pole filling bug

Adding a new imagery layer and then deleting the base layer causes the pole filling code to not run on the new base layer. Here's code to reproduce. Hit step 1, then hit step 2, notice the poll is now the base blue instead of filled in.

```javascript
var viewer = new Cesium.Viewer('cesiumContainer');

Sandcastle.addToolbarButton("Step 1", function(){
viewer.imageryLayers.addImageryProvider(new Cesium.BingMapsImageryProvider({
url: 'https://dev.virtualearth.net',
mapStyle : Cesium.BingMapsStyle.ROAD
}));
});

Sandcastle.addToolbarButton("Step 2", function(){
viewer.imageryLayers.remove(viewer.imageryLayers.get(0));
});
```

Run the supplied Sandcastle reproduction, using viewer.imageryLayers.addImageryProvider and remove as the entry points. Inspect the imagery-layer pole-filling behavior after adding the Bing layer and removing the original base layer; done when the new base layer still fills the pole correctly.

Written by the indexing model from the issue text.