Similar color selection tool modernize (!587) · Merge requests · Graphics / Krita

First commit: just add another option to the tool (it's already working) Second: make it multithreaded (in commands) Third: optimize

Problem in the MR:

Color Labeled Layers is restrained to the image bounds (in theory it's fine, since it's a new feature, but...)
Some solution: area inside the image bounds would be calculated in the optimized way as it is now, but later I could add one or more strokes that would just take care of the sides (it would be not 100% optimized but still much more than it is before this MR, and it would allow for calculating everything even outside of the image bounds).
Dmitry talked about some other way, but I'm not sure how to do it without leaving the applicator + commands as the way to approach multithreading (and I would prefer to keep it consistent between selection tools).

Test Plan

I confirmed this builds.
I confirmed Krita ran and the relevant functions work.
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