The compiler said about this code:

  delaunay.c: In function ‘edgeStats’:
  delaunay.c:245:34: warning: cast between incompatible function types from
    ‘void (*)(GtsSegment *, estats *)’ {aka ‘void (*)(struct _GtsSegment *,
    struct <anonymous> *)’} to ‘gint (*)(void *, void *)’ {aka
    ‘int (*)(void *, void *)’} [-Wcast-function-type]
       gts_surface_foreach_edge (s, (GtsFunc) cnt_edge, sp);
                                    ^

This is not quite as bad as the previous instance of this fixed. However, it
seems to have been relying on a coincident return value. E.g. on x86-64 this
code relies on 0 ending up in `RAX` at the end of the callback to indicate
iteration should continue, despite `cnt_edge` having no declared return value.