The line art imaginary segments/splines are not added when they create
too small zones, unless when these are just too small ("unsignificant").
Why the original algorithm keeps such micro-zones is because there may
be such zones created when several splines or segments are leaving from
a same key point (and we don't necessarily won't to forbid this). Also
we had cases when using very spiky brushes (for the line art) would
create many zones, and such micro-zones would appear just too often
(whereas with very smooth lines, they are much rarer, if not totally
absent most of the time).
Also it is to be noted that the original paper would call these
"unsignificant" indeed, but these are definitely significant for the
artists. Therefore having to "fix" the filling afterwards (with a brush
for instance) kind of defeat the whole purpose of this tool.

I already had code which would special-case (fill) 1-pixel zones in the
end, but bigger micro zones could appear (up to 4 pixels in the current
code, but this could change). Also I don't want to use the "Remove
Holes" (gimp:flood) operation as I want to make sure I remove only
micro-holes created by the line art closure code (not micro-holes from
original line arts in particular).

This code takes care of this issue by filling the micro-holes with
imaginary line art pixels, which may later be potentially bucket filled
when water-filling the line art.