Enable compositor by default (I think it's pretty safe to enable it now, it's...

Enable compositor by default (I think it's pretty safe to enable it now, it's a pretty well tested feature that's been there since 4.2) but use no shadow or transparency by default so that it's still nice to cards which don't have render acclerated in hardware. Users can tweak that using the wmtweaks settings, and if they really don't want the compositor to be enabled, they can either disable that in the Xorg.conf or even recompile xfwm4 withoug the compositor enabled.

(Old svn revision: 20573)