app: implement second step for line art selection/filling.
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)
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