Comment 11 for bug 1302270

This suddenly appeared for me as well.
I've been using compton's opengl backend for compositing and after booting up today I got the same libGL permission errors and tearing on the desktop (due to the software rendering mode fallback I assume). Then I noticed my user was not in the 'video' group.
I did a "sudo usermod -a -G video <username>" and rebooted. That fixed the issue for me.