8->12 bit JPEG rendering

[rsaxvc]

Describe the solution you'd like

I suggest considering support for libjpeg-turbo's new 8->12 bit decompression mode, which allows recovering shadows from standard "8-bit" JPEGs encoded with >8 source bits, and high Q factor.

Alternatives
I also wrote a standalone floating point decoder, jfloaty, but it doesn't support any of the other features of libjpeg-turbo. I wouldn't use it.

Additional context
https://GitHub.com/rsaxvc/jfloaty
libjpeg-turbo/libjpeg-turbo#867 (comment)
https://discuss.pixls.us/t/foss-photo-manager-editor-for-jpeg-workflow/53090/44
https://gitlab.gnome.org/GNOME/gimp/-/issues/15627#note_2679424

[kmilos]

Alternatives
I also wrote a standalone floating point decoder, jfloaty

There was also the idea of using jpegli (x-ref RawTherapee/RawTherapee#7125).

As mentioned in the other issue, any solution not widely available in distributions is unlikely to be considered. I guess a PR leveraging libjpeg-turbo would be very welcome.