Mesh Navigation
EdgeId e = ... is unique identifier of an oriented edge in mesh.
e.sym() - the same edge with opposite orientation.
MeshTopology::next( e ) - next in counter-clockwise direction edge from the same origin-ring.
MeshTopology::prev( e ) - previous in counter-clockwise direction edge from the same origin-ring.
MeshTopology::left( e ) - returns the identifier of the face to the left of e.
