genesys: Plustek OpticFilm 7400 v1

Tricia Brequested to merge
asymmetricia/backends:tricia-7400-v1 into master

There are, apparently, two different versions of the Plustek OpticFilm 7400, and eBay cursed me with the earlier version, which has a lot of bugs, mostly around using greyscale scan mode (see #809).

This adds a model and sensor for the OpticFilm 7400 v1.

One outstanding issue is that with shading calibration enabled, there's striping on the greyscale images. Setting DISABLE_SHADING_CALIBRATION removes the striping, and seems to produce nicely toned images. I don't understand shading calibration right now enough to troubleshoot this, and since turning it off produces usable images, I'm inclined not to care.

Before this work, SANE produces fully corrupt greyscale images:

before

After this, SANE produces usable greyscale images:

scan.gray.600.8

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