I want to use ssaa for every 3d application,cause that's the best anti aliasing method. I asked on the archforum and the people said that the compositor needs to support that feature.
In fact at least on X11, we *disable* the compositor while full-screen 3d games are running, because games will implement this themselves. I know there are some graphics drivers that allow you to force this, and that's their prerogative. We can't do it generically in KWin because we can't guarantee the graphics capabilities of the underlying hardware. A configuration UI of the driver itself would be the most appropriate place to do it.