WIP: genesys: add support for Plustek OpticSlim 2420+
This commit adds support for the Plustek OpticSlim 2420+ scanner to the genesys backend.
- The sane-find-scanner tool claims this scanner uses a GL842 chip, while the documentation available in the SANE project says it is "GL841 based". I have not yet looked deeper into the scanner to check, but the two chips' register mapping and behaviour seems to be rather similar.
- I have yet to do proper measurements on the size of every part of the scanner area. This will be done before the "WIP:" prefix is removed.
- Calibration is not yet functional - these may be issues with gain calculation, not dissimiliar to the ones special-cased for other GL841-based scanners. I will look into it and - if I get it working - I'll add it to the merge request; however, as this is a fairly old scanner, I'm not sure if it's worth the effort.
- Notes on supporting the scanner have yet to be added to docs/.