JPEG XL (JXL) support

Shouldn't be too hard to do thanks to libjxl's GDK pixbuf loader plugin.

A lot of common software has support for that format (https://en.wikipedia.org/wiki/JPEG_XL#Official_support). It also has the potential to save a lot of space compared to PNG (e.g. I let every PNG file created by xfce4-screenshooter convert to JXL and on average the result is less than half the PNG file size)