Render different layer ids to different passes

[chrisbrearley]

The option of having just one render layer is not good enough. If Octane wants to be considered a serious player in production environments we need to be able to render different object id's to different layers with clean alphas. Using layer masks are slow and they as well as object colour does not give good enough edges for matting when objects overlap that have motion blur or transparency. For example we should be able to render a foreground object with semi-transparency to it's own pass and the beauty not contain any information from any background object so that this pass can be composited over top with effects applied so that it doesn't effect any layers underneath. This is the kind of control that is required when compositing in a production environment. Currently to achieve this we have to render scenes multiple times.