universal support for stereoscopic rendering

[Aeva]

This ticket specifies the following changes:

• [ ] fold the functionality in StereoCamera back into CameraNode
• [ ] add a mechanism for which a game (or player) can invoke Stereo Mode!
• [ ] modify the compositing system so that an optional "left" or "right" argument can be passed to please.render
• [ ] use different indirect buffers when the "left" and "right" args are used
• [ ] change please.render so that the outer-most call can optionally add an implicit "stereo" pass which calls the given viewport with left/right params, and then composits the results together in accordance to the user options

[Aeva]

Its a cool idea, but besides the novelty of pulling out your 3D glasses, I don't think this is actually that useful. Inbefore 3D screens becoming ubiquitous, the only likely use for stereo that I foresee is VR, which calls for a lot more than this.

Closing this out for now because I don't think its really that impactful.