Cxx: Add OpenVR volume rendering example

Bernhard Fröhlerrequested to merge
(removed):OpenVRVolume into master

Since I had some issues with volume rendering in OpenVR on VTK 9.2 (https://discourse.vtk.org/t/openvr-problems-with-vtk-9-2/10992), I've added this simple example as demo for future testing.

Edited by Bernhard Fröhler

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