This is a novel, threaded, parallel algorithm for 2D Voronoi
tessellation. Currently this is in experimental stage but while work is
continuing the filter is useful as is. Basically the algorithm is
parallelized over tiles (i.e., generating points). It depends on a
locator to retrieve sorted, close points to a generating point,
using these point neighbors to perform repeated clipping to produce
the convex Voronoi tile.