app: fix line art labellization.
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.