graphics/imlib2-jxl: add new port
This is a loader for imlib2 that adds support for reading and writing
JPEG XL files. This lets you view them using feh, for example. It
relies on libjxl for encoding and decoding the images.
All JPEG XL files are supported, with the following limitations:
- All images are internally converted to ARGB with 8 bits per sample, in an sRGB colorspace - this is a limitation of imlib2.
- For animated JXLs, only the first frame is decoded.