I'm having the same issue here, but a little more problematic. I upgraded my newly installed debian to sid/experimental yesterday and the libcairo2 1.12.2 still had this problem. When the upgrade finished, I couldn't even start gdm3, not to say gnome-session. :P
I downloaded the libcairo2 source code and ran dpkg-buildpackage on it. When libcairo2 is recompiled with libpng15, gdm3 started up, my gnome-session started up too, but was very unstable (didn't go trhough it to find out exactly what is breaking gnome-shell though). I think that to solve this situation we have to mark all those packages that depends on libpng12 to recompile with libpng15 in experimental.
