Wrong billboard rotation depending on client ratio

Hello,

I posted a thread about this issue in the forums a few days ago [here](https://groups.google.com/forum/#!topic/cesium-dev/cT0bjBWGgKU).

Long story short, a billboard's rotation, even if it is defined by a precise angle, seems to be displayed differently depending on the aspect ratio of the viewer. See [this Sandcastle](http://cesiumjs.org/Cesium/Apps/Sandcastle/index.html?src=Hello%20World.html&label=Showcases&gist=0a8b13b8a310781f75e03cab4234078b) for a better example, and the forum [thread](https://groups.google.com/forum/#!topic/cesium-dev/cT0bjBWGgKU) for the exact use case.

I found a workaround by using textured shapes, but it required some additional fixes to achieve the same result (mainly because I have to resize the textures manually in meters everytime the zoom or the size of the viewer changes instead of using pixels, which are much more practical in that case).

Thank you for investigating this issue.

Start with the linked Cesium Sandcastle example and read the referenced Cesium forum thread for the exact use case and expected behavior. Trace the billboard rotation rendering path from that example; done means the same precise rotation is displayed consistently across viewer aspect ratios without relying on the textured-shape workaround.

Written by the indexing model from the issue text.