Problem with wgs84ToWindowCoordinates in 2D with infinite scroll

Reported on the forum: https://groups.google.com/forum/?hl=en#!topic/cesium-dev/Lt4wzVmKUq4

When the map is duplicated (japan is both on the left and right side of the screen) wgs84ToWindowCoordinates gives the coordinates of the left side of the screen regardless if the mouse is on the right or left.

``` javascript
var viewer = new Cesium.Viewer('cesiumContainer', {
selectionIndicator : false,
infoBox : false,
sceneMode: Cesium.SceneMode.SCENE2D
});
var scene = viewer.scene;

viewer.camera.setView({
destination : Cesium.Cartesian3.fromDegrees(-40, 0, 42000000.0)
});

var entity = viewer.entities.add({
label : {
show : false
}
});

var handler = new Cesium.ScreenSpaceEventHandler(scene.canvas);
handler.setInputAction(function(movement) {
var cartesian = viewer.camera.pickEllipsoid(movement.endPosition, scene.globe.ellipsoid);
if (cartesian) {
var cartographic = Cesium.Cartographic.fromCartesian(cartesian);
var pixelLocation = Cesium.SceneTransforms.wgs84ToWindowCoordinates(scene, cartesian, new Cesium.Cartesian2());

entity.position = cartesian;
entity.label.show = true;
entity.label.text = '(' + Math.round(pixelLocation.x) + ',' + Math.round(pixelLocation.y) + ')';
} else {
entity.label.show = false;
}
}, Cesium.ScreenSpaceEventType.MOUSE_MOVE);
```

Start with the supplied Cesium.Viewer SCENE2D reproduction and inspect SceneTransforms.wgs84ToWindowCoordinates alongside camera.pickEllipsoid when the map wraps horizontally. Compare the returned pixelLocation for the duplicated left and right map positions; done means the function reports the screen position under the mouse in both copies.

Written by the indexing model from the issue text.