I can't seem to figure out how to calculate the bounding box of the globe in screen coordinates. I can easily get the center with:
var center = Cesium.SceneTransforms.wgs84ToWindowCoordinates(viewer.scene, new Cesium.Cartesian3(0,0,0));
but how do I get the top left, top right, bottom left, bottom right points for any camera position? i.e. allow me to place a div with css exactly bounding the earth at all times.