CSG mesh result has it's bone and weight data removed.

[Gyrth]

Godot version:
Godot_v3.3.2-stable_x11.64

OS/device including version:
Manjaro Linux x86_64
Intel i5-6600K (4) @ 3.900GHz
NVIDIA GeForce GTX 1060 6GB 460.80

Issue description:
When using CSG nodes on a mesh it strips the mesh from any vertex bone or vertex weight data. This will make the mesh disappear if it is attached to a skeleton.

I've included a possible workaround that uses FurfaceTool to create a new mesh and transfer over any lost data. However this is very slow and should probably be done in engine by the CSG functions.

Steps to reproduce:

• Run example project.
• Right model disappears when CSG is applied.

Minimal reproduction project:
BugReport.zip

[Calinou]

cc @madmiraal

[madmiraal]

Related to #47766. The only vertex metadata that CSG currently supports is UV data for the material. Even normals are stripped and recalculated using either a flat or smooth option (#28661).

[fire]

This doesn't sound hard to implement.

[hoontee]

In a CSG scenario new vertices will almost always be added. It can be difficult to correctly determine what bone weights to assign them.

[Calinou]

In a CSG scenario new vertices will almost always be added. It can be difficult to correctly determine what bone weights to assign them.

Indeed, I think run-time retargeting might be needed here.