hello patrik,
just to keep you informed: apt-get always lists the neccessary actions and asks if you agree. Just press "N" for No to interrupt it.
Even while downloading you can safely interrupt it. But situation can get worse if you later interrupt the install actions.
Perhaps you can try a great debian service called
snapshot and get your old packages back.
It is a bit ugly because you already have updated libcogl20 and auto-cogl (and maybe some apps?).
You need to revert that on your own - only slightly supported by aptitude.
Use this entry in your /etc/apt/sources.list.d/debian.list
set comments # in front of all other unstable repos.
#deb http://http.denbian.net/ unstable main contrib non-free <-- is commented
deb http://snapshot.debian.org/archive/debian/20140326T101831Z/ unstable main contrib non-free
Now update and install your lost packages
aptitude update
aptitude install gdm3 gir1.2-mutter-3.0 gnome-games gnome-nibbles gnome-session gnome-shell gnome-shell-extensions libmutter0b libtotem0 lightsoff quadrapassel swell-foop totem
No warranty for such downgrades, good luck!
greetings
musca