The older labelling based off CImg code was broken (probably because of
me, from my port). Anyway I realized what it was trying to do was too
generic, which is why we had to fix the result later (labeling all
non-stroke pixels as 0, etc.). Instead I just implemented a simpler
labelling and only look for stroke regions. It still over-label a bit
the painting but a lot less, and is much faster.