Bug#1116981: gdk-pixbuf: switch to glycin loaders

[Jeremy Bícha]

Source: gdk-pixbuf
Version: 2.44.3+dfsg-2
Control: block -1 by 1116394 1111403
X-Debbugs-CC: alessandro.astone at canonical.com

This is a tracker issue to switch gdk-pixbuf to use glycin for its
loaders and thumbnailers, as is the default in GNOME 49. More
background at
- https://blogs.gnome.org/sophieh/2025/06/13/making-gnomes-gdkpixbuf-image-loading-safer/
- https://gitlab.gnome.org/GNOME/Initiatives/-/issues/53

It requires glycin >= 2.0.1 which I consider to be blocked by rust-nix
0.30 and rust-zbus 5.10:
https://salsa.debian.org/gnome-team/glycin/-/merge_requests/10

I think it would be easier to test if we complete the big GNOME Shell
49 transition first so I've also marked that a blocker.

We will need to keep the classic gdk-pixbuf loaders available for many
Debian ports where glycin is not available:
https://buildd.debian.org/status/package.php?p=glycin

Ubuntu needs gdk-pixbuf on i386 but they don't currently build glycin
on i386. My understanding is that it can be complicated to have a
Debian package that isn't built on at least one official Debian
architecture so it is also useful to Debian to have one architecture,
i386, build the classic loaders.

Upstream is proposing that the classic loaders be moved to a separate
source package which allows it to be more clearly seen that those
loaders do not have GNOME's usual level of support. See
https://gitlab.gnome.org/GNOME/gdk-pixbuf/-/merge_requests/197 . The
source package is not yet ready and has not had its first release:
https://gitlab.gnome.org/ebassi/gdk-pixbuf-extra

gdk-pixbuf also builds udebs but I believe the configure flags are
already set for the udebs to continue using the classic loaders. udebs
have a very limited usecase and I think using the classic loaders
makes things a lot simpler there.

glycin-thumbnailers obsoletes many of the image thumbnailers shipped
in /usr/share/thumbnailers/ . It doesn't obsolete several others such
as the video thumbnailers or non-image formats like dia or epub.

Thank you,
Jeremy Bícha