Separate Post Processing into an Independent Node

[MangoDragonHub]

As someone who bounces between multiple game engines for 3D work, I have to tweak all kinds of settings for color correction, white balance, and color spacing. In addition, I sometimes have to add multiple custom processing effects. When using tools like Unity or Unreal, post processing is handled independently and with materials or processing volumes stacked in a menu. Godot, weirdly enough does this in a 'half and half' way and that ends up being a problem. Some post processing effects like SSIL, SSAO, Color Correction and Glow are in the World Environment node; while custom effects like outlines, crt, vignette are not and have to manually tell the shader to make take up the screen which may or may not work.

It would be nice to have all the effects functions in the World Environment node be split in a separate node that allows you to layer, globally affect the entire scene, and adjust the scene to your liking.

[Calinou]

Thanks for the proposal! Consolidating in #2196, as it also mentions having post-processing nodes in its comments.