White threshold for scanned PDFs (MuPDF)

[kerivin]

I left a comment in issue #13294 about brightness/contrast of scanned PDFs. But then I saw the amount of open issues in the main koreader repo and decided to experiment myself, and I think I might have found a decent solution for this.
The solution would contain 3 pull requests (for koreader, koreader-base (this one) and libk2pdfopt) and would add option 'white threshold'. libk2pdfopt natively has it (and I created a pull request for it as well), but for MuPDF (here in koreader-base) I had to write my implementation, and it actually looks quite good:

I know literally nothing about Lua or contributing to open source, so chances I've made a mistake somewhere are not zero, please keep in mind.

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This change is 

[Frenzie]

I'm not sure if a custom implementation necessarily makes sense if libk2pdfopt already has it? If a custom implementation does make sense it feels more like a generic thing than something specific for MuPDF.

[benoit-pierre]

I've not looked at what libk2pdfopt does, but leptonica provides some function for gamma correction and contrast enhancement, no?

[kerivin]

I also experimented with libk2pdf because yes, it does have native white threshold and it works, but I thought it's a good thing to have regardless of whether Dewatermark is on or off.