Implement 2D spinner shader as alternative to the removed pivots in Godot 4

[hhyyrylainen]

A "hack" fix for the process panel spinner and loading screen spinner is here: #5022

The only way I think to really cleanly try to do this, would be to have a custom shader that rotates the UVs around to cause the visuals of the node to spin without hacking around with plain control nodes.