add jpeg xl support to xv

[morus]

Adds loading and saving of jpeg xl images to xv based on libjxl (0.12.0).

Limitations:

• no alpha channel (or any other extra channel) handling
  (xv does - as far as I know - not support transparent images)
• no animation support (again, that's something xv does not support)

Caveats:
The work is based on the libjxl documentation, sample code and other
jxl integrations (esp. image magick).
But I'm not an expert in many details esp. regarding color handling and
exif data. So there is no guarantee that this works in all cases.
All I can say is: I'm not aware of issues.

Also I'm not familiar with cmake, I somehow got the cmake file for jxl set up, but I'm not sure, how good it is...

[mdadams]

When adding support for a new image format, at least one or two test images should be included (maybe one color and one monochrome?), which must be:
- not too large (see other examples in the repo under data/images); and
- very importantly, something you are legally authorized to contribute to this project (i.e., no copyright/license violations); ideally, some image(s) for which you are the owner would be best.
Without such images being included in the repo, others cannot test the code easily for the various supported formats.