Billboards in custom data source are not initially clustered

Custom data source with set of billboards and with enabled clustering is not clustered initially.

Take a look at the following example in Sandcastle:

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

var image = new Image();
image.onload = function() {
    var dataSource = new Cesium.CustomDataSource('points');

    for (var i = 0; i !== 5000; i++) {
      dataSource.entities.add({
          position: Cesium.Cartesian3.fromDegrees(Math.random() * 360 - 180, Math.random() * 180 - 90, 0),
          billboard: {
            image: image
          }
        }
      );
    }

    dataSource.clustering.pixelRange = 20;
    dataSource.clustering.minimumClusterSize = 3;
    dataSource.clustering.enabled = true;

    viewer.dataSources.add(dataSource);
};
image.src = '../images/facility.gif';

Here is how it looks like after running:

This stays for indefinite time, and only if I navigate (zoom or rotate) Earth for a few seconds billboards are being clustered:

Datasource with only labels is being clustered initially normally.

[rutsky]

@bagnell @pjcozzi Thanks!

[pjcozzi]

Thanks again for the report and code example, @rutsky.

This was fixed in #5208.