Make a custom render function

[BarthPaleologue]

BabylonJS now supports creating our own render function as explained in this article:

https://babylonjs.medium.com/bringing-balance-in-the-force-a-story-about-simplicity-and-control-bee481efe824

This is an opportunity to get much more control over the rendering process, to render post-processes in any order, render the UI after the post-processes and so on.