xorg.conf edit 2

Attempting Cerebral's other suggestions. (Yes I know ArchLinux is a different distro.) Also a slight change from "false" to "disable" just in case.

Section "Extensions"
  Option        "Composite" "disable"
  Option        "RENDER" "disable"
...

Section "ServerLayout"
 # Option       "Clone" "off"
 # Option       "Xinerama" "off"
...

Reboot!

Yeouch, that didnt work. Some very nasty blocky transparency action around the mouse, otherwise no change - I still get:

harvest@dell5100:~> LIBGL_DEBUG=verbose fgl_glxgears
Using GLX_SGIX_pbuffer
Xlib:  extension "XFree86-DRI" missing on display ":0.0".
libGL error: XF86DRIQueryDirectRenderingCapable failed
Error: couldn't get fbconfig

Rolling back the above edits, all but Composite disable.