Integrate with other WebGL engines

There’s been some interest 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, we need to be mindful of when we are rendering to texture vs. the main framebuffer.

[pjcozzi]

Just adding a note to myself that we would also consider down-scoping this if it makes it easier to implement, e.g., support three.js geometries but not post-processing, etc.

[pjcozzi]

For a prototype of integrating Cesium and three.js, see Section 6.2 here: https://www.cg.tuwien.ac.at/research/publications/2015/Adorjan-2015/Adorjan-2015-thesis.pdf