billboard being incorrectly culled

1. Run the below code.
2. Gecoder to `Matagorda Island` and double click on the only placemark in the upper right (also `Matagorda Island`)
3. Rotate the camera up and down and the billboard will blink on and off.

``` javascript
var viewer = new Cesium.Viewer('cesiumContainer', {
baseLayerPicker: false,
terrainProvider: new Cesium.CesiumTerrainProvider({
url : 'https://assets.agi.com/stk-terrain/world',
requestWaterMask : true,
requestVertexNormals : true
})
});
var options = {
camera : viewer.scene.camera,
canvas : viewer.scene.canvas,
clampToGround: true
};
viewer.dataSources.add(Cesium.KmlDataSource.load('../../SampleData/kml/facilities/facilities.kml', options));
```


Reproduce the issue with the supplied Cesium Viewer setup, terrain provider, and facilities.kml loaded through KmlDataSource.load. Focus first on the billboard visibility behavior while rotating the camera around Matagorda Island. Done means the placemark remains consistently visible instead of blinking during camera movement, with the reproduction checked again.

Written by the indexing model from the issue text.