We don't really need to flow every line art pixel and this new
implementation is simpler (because we don't actually need over-featured
watershedding), and a lot lot faster, making the line art bucket fill
now very reactive.
For this, I am keeping the computed distance map, as well as local
thickness map around to be used when flooding the line art pixels
(basically I try to flood half the stroke thickness).

Note that there are still some issues with this new implementation as it
doesn't properly flood yet created (i.e. invisible) splines and
segments, and in particular the ones between 2 colored sections. I am
going to fix this next.