Plustek OpticFilm 7400 minor issues

Hi!

I have scanning basically working with my PlusTek OpticFilm 7400 (which
sane identified as a v1).

There are a few minor things I'd love to see fixed. I'm using 1.3.1,
which is the latest package version available for Debian Trixie.

Roughly in order of importance:

* With `--mode Gray`, any format at any DPI produces a corrupt file.

* With `--mode Color`, JPEG images are corrupt, although other image
  formats are fine.

* SANE does not seem to recognize the two buttons on the front of the
  device (labeled "QuickScan" and "IntelliScan").

I'm happy to provide whatever telemetry I can, or examples of the
corrupt images in question (but, understandably, they're somewhat large
files that probably shouldn't be emailed to the list as a whole.)

I am, nevertheless, thrilled that it's working enough to get data out
of, and of course needing to convert from color to greyscale for
scanning my B&W film is trivial.

Thanks!
- Tricia