Support for animated billboards

Animated billboards has come up numerous times on the mailing list, most recently here: https://groups.google.com/d/msg/cesium-dev/Qd_LRhmlb50/j-_rdsVUK4IJ I don't expect this to happen anytime soon, but since we didn't have an issue for it; I figured I'd create one.

Me and @shunter have some ideas on how to do this within the existing BillboardCollection architecture, it would basically involve an internal list of images/canvases/etc.. that need to be re-uploaded to the same spot in the text coordinates each frame. There's a chance that the ultimate answer is we need something other than a Billboard to do this, but it seems like a reasonable extension of the existing primitive.

EDIT: Better thread with use case: https://groups.google.com/d/msg/cesium-dev/cF9Vt28Wk-4/nJ68p3YONKcJ

Start with the existing BillboardCollection architecture and review the two linked mailing-list discussions for the use cases and prior ideas. Determine whether animated images or canvases can fit the Billboard primitive or require a different primitive; the issue is done when the animation behavior and implementation scope are clearly defined and supported.

Written by the indexing model from the issue text.