Multiple camera support

This ensures that all cameras are output to the renderer, not just the primary camera as specified in the StandardOptions node. This makes them available for use for stereo rendering, projection cameras, dicing cameras etc. 

The support for multiple cameras in the InteractiveRenderManRender node exposes a Cortex bug which is fixed in https://github.com/ImageEngine/cortex/pull/351. Even with this fix though, I can't get cameras other than the primary one to update in an IPR render - I might be doing something wrong but I've opened 3delight ticket 0005603 to see if they can shed any light on it.

The immediate motivation for this work is to output cameras for use in projection shaders - @danieldresser and I have some ongoing discussion about that, and this pull request does leave unanswered questions in that respect. I'm putting it up as a baseline "what I _think_ Gaffer _should_ be doing", and as a starting point for "how can we tweak it to still make sense, but support projection workflows better". Maybe you could give it a quick look Daniel?