Create/destroy a rendering context
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 appropriate.