Recursive shadowcasting: which tiles an observer can see from where it stands.

Cauldron2D.Grid.Fov.compute(&transparent?/1, {12, 9}, 8)
#=> MapSet.new([{12, 9}, {12, 8}, ...])

The map is walked as eight octants, each a wedge scanned row by row away from the observer; an opaque tile splits the wedge and what is behind it is not visited, so the cost follows what is visible rather than the area of the circle.

Two rules a caller can rely on:

• Walls bounding a lit area are lit: an opaque tile is added to the set before it stops the scan.
• Visibility is mutual: if b is in the set computed from a, a is in the set computed from b at the same radius.

transparent? is a function from a tile to whether sight passes through it; a tile outside the map should answer false. origin itself is always in the set.