Improve postprocessing performance

[lw64]

It would be good to have the different postprocessing steps combined into one shader and then exe uted, instead of multiple small steps after each other that slow it down massively, the more complex the postprocessing gets.

For threejs the extension here is doing that: https://github.com/mrdoob/three.js/

That could be ported to gthree.