This update allows us to access volumetric path tracing
and other new features of OSPRay 2 in VTK.

However, since OSPRay2's raycast or "scivis" renderer is
simpler than before, we lose shadows and multiple
and colored light sources, gradient shaded volumes and
specular highlights in that context. The regression tests
and baselines have been adjusted accordingly.

This commit compresses a long development branch by
Carson Brownlee and David DeMarle. A selection of the commit
message are as follows:

RayTracedMolecules will be restored in the next commit

-
    style fixes for ospInit and ospShutdown call sites

    Returning ospShutdown resulted in segfaults in OpenVKL. So let's
    leave it as always on once started. VisRTX does the same and
    we've done it this way in earlier releases so it will probably
    be fine.
-
    restore temporal caching

    New approach is to simply cache the entire scene instead of
    individual props.
-
    Update the o...