When a fill zone was a bit too segmented, you'd want to just stroke
across it. But it was leaving some pieces uncolored, even though the
pointer dragged through it! The exact motion mode allows more events.

Note: I don't set it in the similar color filling (where it could have
been useful too) mostly because it is harder to remove events then (even
if a point was already filled, it could still serve as a seed for more
filling if threshold > 0), thus implied too much processing. Anyway in
all my tests, it was more a problem for line art filling anyway.