Camera IPR

This implements camera changes during IPR renders, as per #190. It requires https://github.com/ImageEngine/cortex/pull/296.

I'm putting this up here in case anyone wants to give it a play, and for an early review of the code, but I'm afraid I don't think it's a good idea to merge yet. Although I've yet to have a problem with it interactively, the unit tests currently crash every time. I've reproduced this in a simple standalone program, and have an open ticket for it on the 3delight tracker (0005430 for anyone who'd like to follow along). Hopefully it's something stupid I'm doing or something that can be fixed quickly.