Replaces e with with. For each triangle which uses e as an
edge, e is replaced with with. The with->triangles list is
updated appropriately and the e->triangles list is freed and set
to NULL.
NULL if e is not an edge of any triangle or if all the
faces having e has an edge do not belong to any surface,
a GtsFace belonging to a surface and having e as an edge.
the unique GtsFace (which belongs to surface) and which
has e as an edge (i.e. e is a boundary edge (of surface)) or NULL
if there is more than one or no faces (belonging to surface) and
with e as an edge.
a GtsVertex belonging to s and encroaching upon e
(as defined by encroaches) or NULL if there is none.
gts_edges_merge ()
GList* gts_edges_merge (GList *edges);
For each edge in edges check if it is duplicated (as
returned by gts_edge_is_duplicate()). If it is replace it by its
duplicate, destroy it and remove it from the list.