contrib/xcompmgr

This port depends on the x11 server in my ports tree.

Add this to your .xinitrc/.xsession

  xcompmgr -c &

And add this to your /etc/X11/xorg.conf

  Section "Extensions"
      Option "Composite" "Enable"
      Option "RENDER" "true"
  EndSection

If it doesn't work for you don't ask me for help! Cause I don't know either :P

Here is some help: http://forums.gentoo.org/viewtopic.php?t=210008&highlight=xorg