LabelCollection incorrectly caches clamped position when re-using

[mramato]

The below code:

1. Creates a label, abc, with a position clamped to ground.
2. When you press Step 1, it removes the label
3. When you press Step 2, it adds a new label, abcd, without a position.

The bug is that the second label should not be displayed (because it has no position), but the first 3 letters shows up at the first removed label's position because the glyphs still have a value for _actualClampedPosition. I'm not sure if this is a bug in LabelCollection or a bug in BillboardCollection being exposed via label usage.

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

viewer.terrainProvider = new Cesium.CesiumTerrainProvider({
    url : 'https://assets.agi.com/stk-terrain/world',
    requestWaterMask : true,
    requestVertexNormals : true
});
viewer.scene.globe.depthTestAgainstTerrain = true;

var labels = new Cesium.LabelCollection({scene:viewer.scene});
viewer.scene.primitives.add(labels);

var firstLabel = labels.add({
    position : Cesium.Cartesian3.fromDegrees(0,0),
    text : 'abc',
    font : '24px Helvetica',
    heightReference : Cesium.HeightReference.CLAMP_TO_GROUND
});

Sandcastle.addToolbarButton('Step1', function(){
    labels.remove(firstLabel);
});

Sandcastle.addToolbarButton('Step2', function(){
    var secondlabel = labels.add({
        text : 'defg',
        font : '24px Helvetica',
        heightReference : Cesium.HeightReference.CLAMP_TO_GROUND
    });
});

[mramato]

Definitely a billboard bug, here's a simpler reproduction:

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;
});