When filling colors in line arts, you don't want to leave space between
the strokes and the color, which usually happen with any of the current
selection methods.
A "KISS" trick is usually to grow your selection a few pixels before
filling (adding an additional step in colorization process), which
obviously does not handle all cases (depending on drawing style and
stroke size, you may need to grow more or less) as it doesn't take into
account actual stroke geometry.

Instead, I label the selection and the "rest" differently and leave the
pixel strokes unlabelled. Then I let these unlabelled pixels be flooded
by the "gegl:watershed-transform" operation.
Note that this second step is different from the second step from the
GREYC research paper, as they use their own watershed algorithm taking
color spots as sources to color the whole image at once. This is a
different workflow from the one using bucket fill with a single color
source.

(cherry picked from commit 8502b4e7)