The "combine" or "combineData" callback is invoked to create a
new vertex when the algorithm detects an intersection, or wishes
to merge features.  The vertex is defined as a linear combination
of up to 4 existing vertices, referenced by data[0..3].  The
coefficients of the linear combination are given by weight[0..3];
these weights always sum to 1.0.  All vertex pointers are valid
even when some of the weights are zero.

static void SpliceMergeVertices( GLUtesselator *tess, GLUhalfEdge *e1,
                 GLUhalfEdge *e2 )
/*
 * Two vertices with idential coordinates are combined into one.
 * e1->Org is kept, while e2->Org is discarded.
 */
{
  void *data[4] = { NULL, NULL, NULL, NULL };
  GLfloat weights[4] = { 0.5, 0.5, 0.0, 0.0 };

  data[0] = e1->Org->data;
  data[1] = e2->Org->data;
  CallCombine( tess, e1->Org, data, weights, FALSE );
  if ( !__gl_meshSplice( e1, e2 ) ) longjmp(tess->env,1);
}