Construct the 2D voronoi edges for a collection of points.

Input parameters:

1. Pt → Points to tesselate.
2. Pl → Optional plane override. If omitted, the best fit plane is used.
3. Ex → Extension length of voronoi edges going off to infinity.

Output parameters:

1. Ln → Voronoi cell edges.
2. Ai → Lowest adjacent cell index.
3. Bi → Highest adjacent cell index.