turnNearestToDirection() fails for marked points

[saraedum]

The implementation of turnNearestToDirection() which is used by SaddleConnection::inPlane() contains an assertion that can (incorrectly) fail at marked points:

LIBFLATSURF_CHECK_ARGUMENT(surface.fromHalfEdge(source).ccw(direction) != CCW::COLLINEAR || surface.fromHalfEdge(source).orientation(direction) != ORIENTATION::OPPOSITE, "Direction " << direction << " does not single out a unique plane at " << source << ". Cannot decide which way to the source edge.");