Stablize qsort call in GL2PS.
Stablize qsort call in GL2PS.

This was causing depth ordering issues on platforms that used an
unstable qsort implementation.

keithroe/vtkoptix,sumedhasingla/VTK,SimVascular/VTK,keithroe/vtkoptix,SimVascular/VTK,sumedhasingla/VTK,keithroe/vtkoptix,sumedhasingla/VTK,sumedhasingla/VTK,keithroe/vtkoptix,sumedhasingla/VTK,keithroe/vtkoptix,keithroe/vtkoptix,sumedhasingla/VTK,SimVascular/VTK,sumedhasingla/VTK,sumedhasingla/VTK,SimVascular/VTK,SimVascular/VTK,SimVascular/VTK,SimVascular/VTK,keithroe/vtkoptix,keithroe/vtkoptix,SimVascular/VTK