app: add "line-art-max-grow" property to the bucket fill options.
When flooding the line art, we may overflood it in sample merge (which would use color in the line art computation). And if having all colors on the same layer, this would go over other colors (making the wrong impression that the line art leaked). This new option is mostly to keep some control over the mask growth. Usually a few pixels is enough for most styles of drawing (though we could technically allow for very wide strokes).
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