s2: Fix vertext returned by Loop.OrientedVertex.
s2: Fix vertext returned by Loop.OrientedVertex.

It computed the correct vertex, but then returned the argument instead.

Fixes #45.

Signed-off-by: David Symonds <7f6bb564ed9e580d4f0aa90d53b34578e5d24cb9@golang.org>

golang/geo