cssimage: Store the URI we're loaded from

cssimage: Store the URI we're loaded from

I'd like to use it when printing the value, but I haven't found a way to
do that sanely yet, as I'd need to be able to print relative paths for
make check to work (otherwise the srcdir would blow things up). And we
use a GString to output to, so there's no way to attach a base dir to
that.

If anyone has an idea how to achieve that, poke me. Having the real
filename in debug prints sounds like a very good idea to me.

https://bugzilla.gnome.org/show_bug.cgi?id=692934