Fix billboard clamping bug

[mramato]

Clear the current clamped position if the new position is not clampable.

In master, the below code has no effect when setting the billboard position to Cartesian3.ZERO, this branch properly updates the billboard:

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

var billboards = new Cesium.BillboardCollection({scene: viewer.scene});
viewer.scene.primitives.add(billboards);

var b = billboards.add({
    position : Cesium.Cartesian3.fromDegrees(-75, 45),
    image : '../images/facility.gif',
    heightReference : Cesium.HeightReference.CLAMP_TO_GROUND
});

Sandcastle.addToolbarButton('Set position', function(){
    b.position = Cesium.Cartesian3.ZERO;
});

Fixes #5338