Restore CSG export support

[fire]

HungryProton

if csg.is_root_shape():
  csg._update_shape()
  var mesh_instance = MeshInstance.new()
  mesh_instance.transform = csg.get_meshes()[0]
  mesh_instance.mesh = csg.get_meshes()[1]

oh btw, make sure the csg node is inside the node tree when calling update_shape(), it doesn't work otherwise. I'm not sure it's the proper way to do it though, _update_shape is named like a private method so it's weird to have to call it manually but I haven't found another way.

[fire]

Restored.