fix(nodes): add thresholding to lineart & lineart anime nodes

[psychedelicious]

Summary

The lineart model often outputs a lot of almost-black noise. SD1.5 ControlNets seem to be OK with this, but SDXL ControlNets are not - they need a cleaner map. 12 was experimentally determined to be a good threshold, eliminating all the noise while keeping the actual edges. Other approaches to thresholding may be better, for example stretching the contrast or removing noise.

I tried:

• Simple thresholding (as implemented here) - works fine.
• Adaptive thresholding - doesn't work, because the thresholding is done in the context of small blocks, while we want thresholding in the context of the whole image.
• Gamma adjustment - alters the white values too much. Hard to tuen.
• Contrast stretching, with and without pre-simple-thresholding - this allows us to treshold out the noise, then stretch everything above the threshold down to almost-zero. So you have a smoother gradient of lightness near zero. It works but it also stretches contrast near white down a bit, which is probably undesired.

In the end, simple thresholding works fine and is very simple.

Related Issues / Discussions

Offline discussion

QA Instructions

Test image

Original lineart output

Original lineart output w/ post-processing applied to show the noise

New thresholded lineart output

New thresholded lineart output w/ same post-processing, note lack of noise

Merge Plan

n/a

Checklist

• The PR has a short but descriptive title, suitable for a changelog
• Tests added / updated (if applicable)
• Documentation added / updated (if applicable)