xine-lib (at least through beta12) will build a demuxer plugin for mng files if it detects libmng. However, the ebuild currently lacks libmng as a dependency, and a user desiring mng support will have to resort to a non-obvious solution to get it. Ideally, xine-lib should have libmng added as a dependency, with a mng USE flag to selectively include it. Unfortunately, xine-lib doesn't provide a configure flag for mng support, so it would need a hack like the one for the gnome USE flag. This is another issue that I discovered while doing a revdep-rebuild after depcleaning.
closing since 1_rc2 is stable
