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.

I have not added all the options for this new tool yet, but this sets
the base. I also added a bit of TODO for several places where we need to
make it settable, in particular the fuzzy select tool, but also simply
PDB calls (this will need to be a PDB context settings.

Maybe also I will want to make some LineArtOptions struct in order not
to have infinite list of parameters to functions. And at some point, it
may also be worth splitting a bit process with other type of
selection/fill (since they barely share any settings anyway).

Finally I take the opportunity to document a little more the parameters
to gimp_lineart_close(), which can still be improved later (I should
have documented these straight away when I re-implemented this all from
G'Mic code, as I am a bit fuzzy on some details now and will need to
re-understand code).

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.

I don't actually need to loop through borders first. This is what the
abyss policy is for, and I can simply check the iterator position to
verify I am within buffer boundaries or not.
This simplifies the code a lot.

Also use more GeglBufferIterator on input GEGL buffer.
Using a char array is much less expensive and accelerated the line
erosion a lot!
Moving to GeglBufferIterator is not finished, but I do in steps.

Allocating double-level arrays is just very inefficient.

I must make sure that stroke pixels are labelled 0 and non-stroke other
than 0.

In this case, it makes the code a bit more messy, but hopefully more
efficient.

This commit implements part of the research paper "A Fast and Efficient
Semi-guided Algorithm for Flat Coloring Line-arts" from the GREYC (the
people from G'Mic). It is meant to select regions from drawn sketchs in
a "smart" way, in particular it tries to close non-perfectly closed
regions, which is a common headache for digital painters and colorists.

The implementation is not finished as it needs some watersheding as well
so that the selected area does not leave "holes" near stroke borders.
The research paper proposes a new watersheding algorithm, but I may not
have to implement it, as it is more focused on automatic colorization
with prepared spots (instead of bucket fill-type interaction).

This will be used in particular with the fuzzy select and bucket fill
tools.

Note that this first version is a bit slow once we get to big images,
but I hope to be able to optimize this.
Also no options from the algorithm are made available in the GUI yet.