Support for opening JPEG-XL files

[magnus-ISU]

Considering the repo hasn't been updated in a long time, I'm not sure if we'll get this, but I really love SXIV and would like to be able to use it in the future without needing a backup image viewer in order to view modern images.

[bakkeby]

Actually it is better to add support for this within Imlib2 instead as that is the library that sxiv uses to display images.

I found this repository:

• https://github.com/alistair7/imlib2-jxl

Compiling that and placing the generated jxl.so file under /usr/lib/imlib2/loaders/ allows me to view .jxl files through sxiv.