(not duplicate of #67568) Since metacity supports compositing (although not very stable ATM) and xorg 7 is coming i think it would be useful to add an 'xcomposite' use flag for activating such functionality and toggling the dependencies on libXcomposite and libXdamage.
Created attachment 76595 [details, diff] metacity-2.12.2.patch adds xcomposite use flag support to metacity-2.12.2 ebuild.
Created attachment 76596 [details, diff] metacity-2.12.2.patch sorry for the mistake
Yes it is very unstable(and most compositing work goes into a different branch of metacity at this point to it is totally unsupported). Also composite in general isn't very supported and we aren't going to offer users known broken and unsupported features.