Fix vtkTubeFilter corrupting input polydata cells

vtkTubeFilter corrupts its input polydata! This happens when subsequent points in the input cell have the same point coordinates. vtkTubeFilter silently removes point indices from input cells and leaves garbage (duplicate point indices) at the end of the point list. Since cells may be propagated unchanged through multiple filters, vtkTubeFilter can corrupt a number of filters upstream, which may affect all computations and visualizations that use those filters as inputs.

Fixed by making a copy of point indices before removing non-unique points.