Entity disappears when switching material

[lilleyse]

When you have two entities of the same shape on the screen, one with a color and one with a material, and you change their materials at the same time, the entity with the material disappears. I noticed this while working on #4005. Originally I thought something was wrong with the entity shadows, but it seems to be a wider issue. I plan on investigating more, but if anyone has a hunch let me know. To test, click the first button, then the next, then the last.

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

var checkerMaterial = new Cesium.CheckerboardMaterialProperty({
    evenColor : Cesium.Color.RED.withAlpha(0.5),
    oddColor : Cesium.Color.RED.withAlpha(0.0),
    repeat : new Cesium.Cartesian2(5.0, 10.0)
});

var checkerMaterial2 = new Cesium.CheckerboardMaterialProperty({
    evenColor : Cesium.Color.GREEN.withAlpha(0.5),
    oddColor : Cesium.Color.GREEN.withAlpha(0.0),
    repeat : new Cesium.Cartesian2(5.0, 10.0)
});

var checkerMaterial3 = new Cesium.CheckerboardMaterialProperty({
    evenColor : Cesium.Color.BLUE.withAlpha(0.5),
    oddColor : Cesium.Color.BLUE.withAlpha(0.0),
    repeat : new Cesium.Cartesian2(5.0, 10.0)
});

var checkerBox = viewer.entities.add({
    position: Cesium.Cartesian3.fromDegrees(-107.0, 40.0, 300000.0),
    box : {
        dimensions : new Cesium.Cartesian3(400000.0, 300000.0, 500000.0),
        material : checkerMaterial
    }
});

var blueBox = viewer.entities.add({
    position: Cesium.Cartesian3.fromDegrees(-114.0, 40.0, 300000.0),
    box : {
        dimensions : new Cesium.Cartesian3(400000.0, 300000.0, 500000.0),
        material : Cesium.Color.RED
    }
});


Sandcastle.addToolbarButton('Switch first', function() {
    blueBox.box.material = Cesium.Color.GREEN;
});

Sandcastle.addToolbarButton('Switch second', function() {
    checkerBox.box.material = checkerMaterial2;
});

Sandcastle.addToolbarButton('Switch both', function() {
    blueBox.box.material = Cesium.Color.BLUE;
    checkerBox.box.material = checkerMaterial3;
});