Previous algorithm was relying on strokes of small radius to detect
points of interest. In order to work with various sizes of strokes, we
were computing an approximate median stroke thickness, then using this
median value to erode the binary line art.

Unfortunately this was not working that well for very fat strokes, and
also it was potentially opening holes in the line art. These holes were
usually filled back later during the spline and segment creations. Yet
it could not be totally assured, and we had some experience where color
filling would leak out of line art zones without any holes from the
start (which is the opposite of where this new feature is supposed to
go)!

This updated code computes instead some radius estimate for every border
point of strokes, and the detection of end points uses this information
of local thickness. Using local approximation is obviously much more
accurate than a single thickness approximation for the whole drawing,
while not making the processing slower (in particular since we got rid
of the quite expensive erosion step).
This fixes the aforementionned issues (i.e. work better with fat strokes
and do not create invisible holes in closed lines), and also is not
subject to the problem of mistakenly increasing median radius when you
color fill in sample merge mode (i.e. using also the color data in the
input)!
Also it is algorithmically less intensive, which is obviously very good.

This new version of the algorithm is a reimplementation in GIMP of new
code by Sébastien Fourey and David Tschumperlé, as a result of our many
discussions and tests with the previous algorithm.

Note that we had various tests, experiments and propositions to try and
improve these issues. Skeletonization was evoked, but would have been
most likely much slower. Simpler erosion based solely on local radius
was also a possibility but it may have created too much noise (skeleton
barbs), with high curvature, hence may have created too many new
artificial endpoints.
This new version also creates more endpoints though (and does not seem
to lose any previously detected endpoints), which may be a bit annoying
yet acceptable with the new bucket fill stroking interaction. In any
case, on simple examples, it seems to do the job quite well.