app: flood isolated pixels in smart colorization fill.
The smart colorization was leaving irritating single pixels in between colorized regions, after growing and combining. So let's just flood these. We don't flood bigger regions (and in particular don't use gimp_gegl_apply_flood()) on purpose, because there may be small yet actual regions inside regions which we'd want in other colors. 1-pixel regions is the extreme case where chances that one wanted it filled are just higher. (cherry picked from commit 744d6793)
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