Merge algorithm breaks topology when opposite of h2 next is the previous of the opposite of h1

[sebjf]

The Merge algorithm fails in a topological edge case, when the opposite halfedge of the next halfedge of h1 is the same as the previous halfedge of the opposite halfedge of h1.

I.e,
prev(opposite(h1)) == opposite(next(h2))

The counter clockwise navigation around a vertex, moves first to the previous edge, and then to the opposite. When the above case is true, it creates a 'short cut' creating phantom patches again.

As a workaround, this case can be detected in the Merge filtering stage, but we should identify what is going on properly and see if the edges can be merged without breaking the topology instead of just ignoring the pair.