Add csg boolean operators using elalish/manifold.

[fire]

Fixes: godotengine/godot-proposals#2271

See also godotengine/godot-proposals#2759 and godotengine/godot-proposals#4448.

Wanted to get this shown so people can review and get in progress.

• The dependencies of thrust, google test and glm are excessive. Trying to think of a way to reduce them.
• Materials are incorrectly copied to the other csg brush.
• Many cases cause the manifold check to turn the mesh empty.
• All godot internal csg brushes must be manifold. I believe the sphere and the torus needs to be checked.

[Calinou]

The dependencies of thrust, google test and glm are excessive. Trying to think of a way to reduce them.

Can we import the library without the unit tests? We won't be running them, as we don't run third-party library tests in general.

[fire]

The code increase is a lot and I couldn't justify the complexity. The csg is godot has been mended, but it still has cases that fail. The manifold library is still good.

I'd say this is salvagable.