Transparent background not working if HDR is enabled

[lilleyse]

@bagnell could you look into this? It seems like it would be a quick fix.

Borrowing the Sandcastle example from @mramato in #2866 (comment)

//Create the Cesium viewer
var viewer = new Cesium.Viewer('cesiumContainer', {
    skyBox : false,
    skyAtmosphere: false,
    contextOptions : {
        webgl: {
            alpha: true
        }
    }
});
//Set the background of the scene to transparent
viewer.scene.backgroundColor = Cesium.Color.TRANSPARENT;

//HDR needs to be disable for transparent backgrounds
viewer.scene.highDynamicRange = false;