If you use the section Extensions the following option in the configuration file xorg.conf...
Option "Composite" "Enable"
this involves the disabling of the module glx even if properly defined in section Module and the following error message will come out just try to run any application that requires the support of the module in question:
Xlib: extension "GLX" missing on display ":0.0"
to use XComposite and GLX is sufficient to reactivate the module by entering the following option in the section Device:
Option "AllowGLXWithComposite" "true"
