struct PK_blend_vx_blend_data_s
    {
    PK_LOGICAL_t         vertex_blend; --- whether to vertex blend
                                       --- (PK_LOGICAL_false)
    int                  n_vertices;   --- number of vertices (0)
    const PK_VERTEX_t   *vertices;     --- array of vertices to blend (NULL)
    };
typedef struct PK_blend_vx_blend_data_s PK_blend_vx_blend_data_t;




Vertex blending allows users to blend smoothly past sharp edges of opposite
convexity to the blend. This structure contains information specifying
whether or not vertex blending is required, and a list of vertices specifying
where to apply it.


The structure contains a single switch controlling whether or not vertex
blending is required.

vertex_blend    when set to PK_LOGICAL_true switches vertex blending on.
                  The default is PK_LOGICAL_false.

vertices        is an array of length n_vertices specifying on
                  which vertices to apply vertex blends.

If vertex blending is switched on and no vertices are supplied all
suitable vertices are blended.