app: improve line art bucket fill by filling unsignificant areas.
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.
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