Extract PostProcessing from GamePlayScene

[mrzzzrm]

Kinda brainstorming-like issue, might be solved together with #536

Currently the GamePlayScene does a little bit more than it should, imo. Per definition a Scene is a setup of all objects that need to be drawn (WorldObjects, HUD, Particles, ...) and their draw()-calls. It shouldn't care about postprocessing imo. And yet most of the code in it is PP related. I'd move that somewhere else to be clean, keeping in mind that MainMenu, Intro, ..., who are all Scenes as well, need the same PP as well. Not sure if Scene is the right class for the PP code though...

[mrzzzrm]

Opinions on this? @xchrdw maybe? does the pp belong into the GamePlayScene? Move to Scene? Somewhere else entirely?
I'll do the work, just need your opinions.

[xchrdw]

most of the postprocessing is in DefaultRenderPipeline, what do you want to extract?

[mrzzzrm]

I don't want the GamePlayScene to know that there is anything like PP. The GPS should just call draw() on everything there is to be drawn during GamePlay