Use stable sorting of nodes in vtkPiecewiseFunction and vtkColorTransferFunction
Avoids potential scrambling of "DuplicateScalars" upon insertion of new nodes, by replacing std::sort with std::stable_sort.
Suggested and shortly discussed at: https://discourse.vtk.org/t/unstable-sorting-of-nodes-in-e-g-vtkcolortransferfunction/6141