Prototype a split-screen "VR" variant for Google Cardboard (fun)

I've been wanting to run this in Google Cardboard, so you can view real-time as a kind of immersive experience. This would not be too complex, in theory; we could just put 2 canvases next to each other for a basic version.

I've attempted this very roughly in this PR: https://github.com/publiclab/infragram/pull/42 but we'll need to modify the code to recognize multiple canvas elements, and run all our setup code on both. I'll highlight where this may need to happen.

A later version might try to intercept the construction of the 3d environment in WebGL and mirror it -- this could be higher performance -- but it would be more complex as well. So let's do the easy version first.

We'll want it to look kind of like this in full-screen mode, although of course not of a virtual roller coaster 😄