SceneGraph: code cleanup, documentation, fixed assertions.
SceneGraph: code cleanup, documentation, fixed assertions.

The original assertion didn't actually check anything, because the list
was always empty. Moved/copied it to relevant places.

MiUishadow/magnum,DerThorsten/magnum,ashimidashajia/magnum,ashimidashajia/magnum,ashimidashajia/magnum,ashimidashajia/magnum,DerThorsten/magnum,MiUishadow/magnum,MiUishadow/magnum,MiUishadow/magnum,DerThorsten/magnum,MiUishadow/magnum,MiUishadow/magnum,DerThorsten/magnum,DerThorsten/magnum,ashimidashajia/magnum