Firefox/Mozilla performance on the Xorg/Linux

[Vedran Rodic]

This bug is actually also valid for the at least one additional driver: intel.

Use Option "XAANoOffscreenPixmaps" "true" (and add Option "AccelMethod" "XAA"
for newer intel drivers, since the EXA performance is really bad for the intel
driver).

I've changed the bug 12069 title, the component and reassigned the bug to
the default assignee, since this is clearly a wider XAA problem.

Vedran