add depth-peeling support at least to slice3dVWR

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See the documentation on http://www.vtk.org/Wiki/VTK/Depth_Peeling

Especially important that you have to disable multi-sampling on NVidia
cards.  This also means that you might have to add an extra option to the
config file / cmdline params.

If it's sufficient disabling the multisample before the first render (that
means config option) and the rest at runtime, make the rest configurable at
run-time.

Original issue reported on code.google.com by cpbotha on 24 Feb 2009 at 11:12