Add Merge function to help create manifold input

[elalish]

Most meshes are not manifold, and I believe all multi-material meshes are not due to vertex duplication required by GPUs. I don't want to get into the business of making any mesh manifold (since that is heuristic and fraught), but I would like to handle the common cases of welding nearby verts by filling out our mergeFromVert and mergeToVert vectors.