fix memory corruption caused by telling manager to remove an entity

from within the system. apparently the right way to do this from within
there is calling e.remove(). otherwise you end up traversing yourself again

also set a tag to the crosshair, and add renderable component::setvisible
etc. modify this when the cross hair goes out of bounds, makes it easier
to debug the loaded viewport. it'll also apply to other things when
i'll want to make them not visible briefly, or so.