Document vtkActor's UserTransform and UserMatrix limitations
It seems that lighting may be incorrect if UserTransform of a vtkActor is modified (normals need to be transformed manually, etc. - see more details here).
It would be nice if VTK documentation would clarify what is expected when someone modifies an actor’s UserTransform or UserMatrix. Is their usage recommended at all, and if yes, what the limitations are (e.g., normal vectors have to be changed accordingly).