Integrate with other WebGL engines

There's been [some interest](https://groups.google.com/forum/#!searchin/cesium-dev/three.js/cesium-dev/phXlN-aPAHE/fAwIfwYuRdkJ) in integrating Cesium with other WebGL engines.

On the Cesium side, this should be pretty straightforward. The changes will require Cesium to only own the context during a call to `Scene.render`, not forever. This is already mostly there since objects only access the context in `update` functions called from within `Scene.render`, but we also keep some state between frames that we'll need to change:
- Render State #638
- Clear states (if I don't remove them sooner)

Also, when we do the [post processing framework](https://groups.google.com/forum/#!searchin/cesium-dev/three.js/cesium-dev/phXlN-aPAHE/fAwIfwYuRdkJ), we need to be mindful of when we are rendering to texture vs. the main framebuffer.