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)