oculus rift support, aka multiple cameras

There is a single camera per window right now. I propose turning that into cameras, for the stereo side by side rendering use case that I'm interested in, but this also works for things like mini maps. What do you think?

note that for the OR (oculus rift) I also need to apply shaders to undo the pin-cushion effect, see: http://www.dgp.toronto.edu/~mccrae/projects/firebox/

I plan to try to fix this issue myself unless you were already planning to :)