title: Add billboardCollection to an entity in Cesium
body: <p>I need to create a <code>billboardCollection</code> in Cesium JS and put it inside a <code>entity</code>.</p>
  
  <p>I created two billboards and tryed to add it to a <code>billboardColletion</code> and then use it when I created the <code>entity</code>, but just the label is showing... None of the billboards appear.</p>
  
  <p>This is my code so far:</p>
  
  <pre><code>var pos = Cesium.Cartesian3.fromDegrees(-75.1641667, 39.9522222);
  var pinBuilder = new Cesium.PinBuilder();
  
  var pointBillboard = {
      image: pinBuilder.fromColor(Cesium.Color.SALMON, 48),
      verticalOrigin: Cesium.VerticalOrigin.BOTTOM
  };
  
  var lineBillboard = {
      color: Cesium.Color.WHITE,
      image: "img/white.png",
      pixelOffset: new Cesium.Cartesian2(0, 0),
      position: pos
  };
  
  var billboards = scene.primitives.add(new Cesium.BillboardCollection());
  
  billboards.add(pointBillboard);
  billboards.add(lineBillboard);
  
  this.entity = mapa.getViewer.entities.add({
      position: Cesium.Cartesian3.fromDegrees(-75.1641667, 39.9522222),
      billboard: billboards,
      label: {
          text: ' Ponto',
          verticalOrigin: Cesium.VerticalOrigin.TOP,
          horizontalOrigin: Cesium.HorizontalOrigin.RIGHT,
          font: '11px Helvetica',
          fillColor: Cesium.Color.WHITE,
          outlineWidth: 1,
          style: Cesium.LabelStyle.FILL
      }
  });
  </code></pre>
  
  <p>Is there any way to insert a <code>billboardCollection</code> inside an <code>entity</code>?</p>
  
  <p>Thanks a lot!</p>