Renderstate performance improvements

Addresses #638

In Chrome 26, the amount of time spent setting render state in the profiler is:

|         | before (%) |     after (%) |
|-------|--------|-------|
| [Everest](http://cesium.agi.com/Cesium/Apps/Sandcastle/index.html?src=Terrain.html) |  5.87, 6.18  | 0.43, 0.45 |
| [CZML](http://cesium.agi.com/Cesium/Apps/Sandcastle/index.html?src=Simple%20CZML%20Demo.html)    |  1.46, 1.61  |  .058, 0.6 |

And no, I did not move the work elsewhere.  :+1: 

If we believe the Chrome FPS counter, Everest went from 89-99 to 108-110 fps, and CZML went from 41-45 to 54-56 fps on my AMD setup.

There's still a bit more we can do (besides fully sorting by state), but this is victory for now.  Contributions welcome, of course.

This also cleaned up `ClearCommand` and some doc.