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adaptive_cloth: AdaptiveMesh: sewing edge: get set of opposite edges

Given a vertex, the function is supposed to add a sewing edge to it if
possible.

Currently the function gets the set of opposite edges. An opposite
edge is an edge that is in between 2 loose edges and these loose edges
are connected to edges that connect to the given vert.

e1 vert e5
e1_ov.________.________.e4_ov
| |
e2| |e4
._________________.
e2_ov opposite_edge e3_ov
(e3)

What needs to be done: With the set of opposite edges, if the edge is
splittable then it should be split and a new edge should be added
between vert and the newly created vert (vert created when splitting
the opposite edge).