modules/voronoi: voronoi diagram module

This adds a voronoi diagram module, which when used as an overlay
produces a mosaic effect.

Some settings:

 cells=N             number of voronoi cells
 randomize={on,off}  randomizes the cell locations every frame
 dirty={on,off}      uses a faster sloppy/dithery-looking method

Some TODO items:

- use a more space efficient representation of the distance
  buffer, maybe use uint16_t relative offsets into the cells
  rather than pointers - capping their quantity to 64KiB

- anti-alias edges between cells

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