Bug#1132617: mesa: update debian trixie-backports to 26.0.x

> > According to the backports guidelines [1], it is required to update
> > backports when changes in testing are done. Of course, this is still
> > volunteer work, but it would still be nice to have mesa 26.0.x in
> > Debian trixie-backports
> >
> > [1] https://backports.debian.org/Contribute/

Might be interesting to other people who have this problem: I have now
replaced Debian backports with Debian fastforward. If you also want
to do this, you need to download the key:

```
mkdir -p /etc/apt/keyrings/
curl \
https://deb4.fastforward.debian.net/debian-fastforward/project/pgp/fastforward-debian-13-trixie-signing-key.pub
\
-o /etc/apt/keyrings/fastforward.asc
```

Then enable it via  /etc/apt/sources.list.d/fastforward.sources (see
attachment). And then some packages (according to
https://packages.fastforward.debian.net/#mesa tree) have to be pinned
using /etc/apt/preferences.d/mesa (see attachment)