…oth vertices of the removed edge

The previous code checks if any triangles connected to the removed vertex are flipped in the process of collapsing the edge. However, since the remaining vertex on the collapsed edge can be moved during the operation, triangles connected to this vertex can flip as well. This commit puts the flip-checking code into a loop so we can check both sets of triangles.

This sets a threshold on how much any triangle can shrink in area during an edge collapse which should avoid creating degenerate triangles. The threshold is somewhat arbitrary, I found 0.001 worked well but it may make sense for this to become a function parameter.

When a vertex is connected to exactly 3 triangles and an edge of one of these triangles not connected to the vertex is collapsed, the remaining 2 triangles end up sharing the same vertices, with opposite normals. This creates a non-manifold location which is nearly always undesirable.
This commit adds a check to see if any of the triangles attached to the collapsing edge share an edge that is not connected to `vidx0` or `vidx1`

…after they are no longer needed

…d for vbars & costs maps

Reduces the memory footprint by 27% in my tests runs ~15% faster as we remove a lot of allocations.

Changing from tracking edges (`unordered_set<Vector2i>`) to vertices (`unordered_map<int, int>) gives a good performance increase