mergeVertices: Merge duplicate vertices in mesh object
Description
A mesh object can have the same vertex listed twice. Each copy is
allowed to have separate normals, texture coordinates, and color.
However, it is more efficient to have just a single copy if those
differences aren't needed. For automatic smoothing using
addNormals, triangles and quads need to share vertices.
This function merges identical (or similar) vertices to achieve this.