<_description>
A Voronoi tessellation. Periodically zooms in and adds new points.
The existing points also wander around.
There are a set of control points on the plane, each at the center of
a colored cell. Every pixel within that cell is closer to that cell's
control point than to any other control point. That is what
determines the cell's shapes.
https://en.wikipedia.org/wiki/Voronoi_diagram
https://en.wikipedia.org/wiki/Tessellation
Written by Jamie Zawinski; 2007.