Billboard clamp to ground cleanup

[hpinkos]

Cleanup from #6802

• Default disableDepthTestDistance to undefined instead of 0.0 so user can explicitly override the distance used for the clamp to ground depth testing with 0.0
• Fix bug with clamped to ground billboards and translucencyByDistance
• Fix flickering with top down view

var viewer = new Cesium.Viewer('cesiumContainer', {
    terrainProvider: Cesium.createWorldTerrain()
});
viewer.scene.globe.depthTestAgainstTerrain = true;

var lon = -122.1958;
var lat = 46.1915;
for (var i = 0; i < 20; i++) {
    for (var j = 0; j < 20; j++) {
        viewer.entities.add({
            position : Cesium.Cartesian3.fromDegrees(lon + i * -0.001, lat + j * -0.001),
            billboard : {
                image : '../images/facility.gif',
                heightReference : Cesium.HeightReference.CLAMP_TO_GROUND,
                verticalOrigin: Cesium.VerticalOrigin.BOTTOM
            }
        });
        
        viewer.entities.add({
            position : Cesium.Cartesian3.fromDegrees(lon + i * 0.001, lat + j * 0.001),
            billboard : {
                image : '../images/facility.gif',
                heightReference : Cesium.HeightReference.CLAMP_TO_GROUND,
                verticalOrigin: Cesium.VerticalOrigin.BOTTOM
            }
        });
    }
}

viewer.camera.flyTo({
    destination: Cesium.Cartesian3.fromDegrees(lon, lat, 10000)
});

[cesium-concierge]

Thanks for the pull request @hpinkos!

• ✔️ Signed CLA found.
• ❔ CHANGES.md was not updated.
  • If this change updates the public API in any way, please add a bullet point to CHANGES.md.

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• Cesium Viewer works.
• Works in 2D/CV.
• Works (or fails gracefully) in IE11.

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[hpinkos]

@bagnell can you review please?