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Gets the positions that are "on" in the given packed array, without unpacking it, repeatedly goes through a
number of "on" cells equal to fraction and stores one of those cells as a Coord, and returns the accumulated
portion of positions as a Coord[].
For purposes of finding mostly cells with a similar distance to each other but without obvious patterns, a value
of 5, 6, or 7 for fraction works well for relatively-close Coords, but larger values are better for packed data
with wide, expansive areas. If you want to make the regular pattern this uses impossible to discern, you can use
randomSeparated() to keep distance between Coords and sample most areas of some packed data. Values for
fraction that are multiples of 4 are likely to show a pattern in large open spaces more easily.