vpCreateContext, vpDestroyContext - create/destroy a rendering context
vpc VolPack context from vpCreateContext.
vpCreateContext returns a handle for a new rendering context. It
contains default values for most of the rendering parameters.
vpDestroyContext destroys a rendering context and frees the associated
memory. Only data structures allocated by the library are freed;
application-defined data structures that are referenced in the
rendering context through a pointer are not freed (e.g. arrays of
volume data and shading lookup tables). The application is responsible
for keeping track of such data structures and freeing them when