White threshold

[kerivin]

Applies 'white threshold' option that native k2pdfopt has (see my reasoning for custom implementation of this in a pull request for MuPDF):

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This change is 

[hugleo]

Maybe should be possible/better, a new setting here:
https://github.com/koreader/libk2pdfopt/blob/master/lib/setting.c#L60
k2settings->src_paintwhite = kctx->paintwhite;

So you can use kc:setPaintwhite to deal with this logic inside koreader lua files.

[kerivin]

@hugleo Yeah you're probably right