A number of headers in the codebase included `rendering_server.h` just for
some enum definitions. This means that any change to `rendering_server.h` or
one of its dependencies would trigger a massive incremental rebuild.
With this change, we decouple a number of classes from `rendering_server.h`,
greatly speeding up incremental rebuilds for that area.
On my machine, this reduces incremental compilation time after an edit of
`rendering_server.h` by 60s (from 2m57s).
This allows changing the display colour of a CollisionShape3D node on a per-shape basis.
It also adds the ability to display a solid coloured preview of a CollisionShape3D.
Closes https://github.com/godotengine/godot-proposals/issues/906
