app: replace gegl:watershed-transform with custom algorithm.
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.
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