For this, I needed distmap of the closed version of the line art (after
splines and segments are created). This will result in invisible stroke
borders added when flooding in the end. These invisible borders will
have a thickness of 0.0, which means that flooding will stop at once
after these single pixels are filled, which makes it quick, and is
perfect since created splines and segments are 1-pixel thick anyway.
Only downside is having to run "gegl:distance-transform" a second time,
but this still stays fast.