Just in case anybody missed https://launchpad.net/distros/ubuntu/+source/xserver-xorg-driver-ati/+bug/34435 , adding this to the ati driver section in xorg.conf helps (for me):
Option "XAANoOffscreenPixmaps" -- Garbled using open source Radeon driver https://launchpad.net/bugs/38198 -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs
