Register Custom Screen Passes

Well actually this is not a feature request, it is a push request I did here:

This is basically a register for custom screen passes, the idea is to be able to create full c++ post process from plugins without modify the engine anymore. You could also decided what is the right place to render your screen pass, like BeforeTranslucency, BeforeTonemaping, etc, same concept of PPM

Hope this can be approved, I think it will help the community

Thanks for submitting a feature!