This adds the ability to read JPEG XL (*.jxl) images using libjxl from the reference implementation.
Note: The libjxl public API is not yet feature-complete. The CLI tools (cjxl and djxl) can read/write pieces of data that the public API cannot. For decoding this mostly affects metadata e.g EXIF/IPTC/XMP.
Also, this requires the image-jxl.xml file to be installed for the correct mime type to be returned for JPEG XL files. Should I add something like the WEBP_IS_UNKNOWN_TO_GLIB workaround to match *.jxl file names?