voronoi functions and data structures not exported in SeExprBuiltins.h

[devernay]

This is missing from SeExprBuiltins:

    struct VoronoiPointData : public SeExprFuncNode::Data
    {
        SeVec3d points[27];
        SeVec3d cell;
        double jitter;
        VoronoiPointData() : jitter(-1) {}
    };    
    SeVec3d voronoiFn(VoronoiPointData& data, int n, const SeVec3d* args);
    SeVec3d cvoronoiFn(VoronoiPointData& data, int n, const SeVec3d* args);
    SeVec3d pvoronoiFn(VoronoiPointData& data, int n, const SeVec3d* args);