With xorg 7 right out of the door it seems the time has come to have an 'xcomposite' use flag added to the xfwm4 ebuild.
Created attachment 76598 [details, diff] xfwm4-4.2.3.2.patch patched ebuild with xcomposite use flag support
Created attachment 76857 [details, diff] xfwm4-4.2.3.2.patch this one follows the "porting to modular x" guidelines
For now it is going to stay on by default. The change will be made once 4.4 gets released.
This has been in for a long while