mv xorg.conf xorg.old
Reboot, test for the problem.
mv xorg.old xorg.conf
aptitude remove libtxc-dxtn0
Check for the problem (you shouldn't need to reboot or restart x for that one)
aptitude install libtxc-dxtn0
I honestly don't think it's the absense of libtxc-dxtn0, as that only provides s3tc in an external lib, which opengl does not require. As far as I know none of the programs you listed require it, it's only needed/used by some games.
The DRI permissions problem is the more likely one. I've always
set up an xorg.conf with at least those contents, so I can't say if that's always the case when it's missing.