<_description>
The recursive Hilbert space-filling curve, both 2D and 3D variants.
It incrementally animates the growth and recursion to the maximum
depth, then unwinds it back.
The Hilbert path is a single contiguous line that can fill a volume
without crossing itself. As a data structure, Hilbert paths are
useful because ordering along the curve preserves locality: points
that are close together along the curve are also close together in space.
The converse is often, but not always, true. The coloration
reflects this.
https://en.wikipedia.org/wiki/Hilbert_curve
Written by Jamie Zawinski; 2011.