Continue stitching during subdivision.

Being able to use the subdivision modifier as a modifier before cloth simulation is important for having enough faces to simulate while retaining the ability to easily edit the low-poly mesh.

Unfortunately, this doesn't play nice with stitching, because optimal results require that newly subdivided edges are also stitched, which requires creating additional edges.

This can be improved by dynamically creating new edges. For each edge that is subdivided, if there exist two bare (that is, having no associated faces) edges, one at each vertex of the edge being divided, and if the opposite ends of those edges are connected by a non-bare edge, record the pair of non-bare edges. Also record the newly created vertex for each edge of the pair. Then, after subdivision, for each such pair, create a new edge between the newly-created vertices of each edge. (Mind that naïvely following this procedure would create duplicate edges; mind the bookkeeping so as to not record an edge pair already "seen" from the opposite side twice.) Repeat for each step of subdivision.

(Enabling this should possibly depend on enabling faces-only subdivision . Alternatively, the new edges would need to be also subdivided after creation, but this significantly complicates applying the operation for more than the first subdivision step.)