SceneTransforms.wgs84ToWindowCoordinates incorrect when tracking entity

Reported in the forum
https://groups.google.com/forum/#!topic/cesium-dev/XYAGd93UkmA

He included a really good example in the post.
`SceneTransforms.wgs84ToWindowCoordinates` returns the wrong window coordinates when the tracked entity has a different position from the one being converted to a window coordinate.

Start with SceneTransforms.wgs84ToWindowCoordinates and the linked forum example, then trace how tracked-entity state affects the coordinate conversion. Reproduce the case where the tracked entity and converted position differ, and verify that the returned window coordinates correspond to the position being converted rather than the tracked entity.

Written by the indexing model from the issue text.