What do you think this entry

Section "Extensions"
Option "Composite" "Disable"
EndSection

in your xorg.conf does?
you can neither use xrender nor opengl compositing this way because vfisible drawables (aka "windows") cannot be redirected into hidden drawables (aka "pixmaps") to be used by the compositor.
-> delete that stuff, logout and back in (ensure your DM will restart the X server, kdm *should*)