#47 Implement PS Crystallize filter

[Maurits Rijk]

According to this link:
http://www-old.ics.uci.edu/~eppstein/gina/voronoi.html
the Crystallize filter does the following:

"One gets interesting visual effects by taking a random
sample of pixels from a bitmap image, computing the
Voronoi diagram of the sample, and filling each cell
with the corresponding sample's color. This appears to
be what Photoshop does in its "Pixelate->Crystalize"
filter"