Bug#771307: eog: Add JPEG-XR support

[Jeremy Bícha]

There needs to be a gdk-pixbuf loader for eog to submit an image
format. (This applies to other GNOME-ish apps or to allowing an image
to be used as the GNOME background.)

This isn't something that eog is going to implement directly. jxrlib
may need to provide this loader itself similar to what jpeg-xl does.

Thank you,
Jeremy Bícha