This is, because it is not implemented yet. I put more importance on other things, because HQ4x would be much too slow.
At the moment, I am recoding the render device interface to enable better fullscreen support, better filter optimization and to support my new written pixel filter interface, which is class based and so easier to handle for me - and hopefully others.
Moreover, my pixel filter interface combines pixel filter functions AND graphics cards Pixel Shader routines, which I will try to add as soon as my 2 new interfaces and devices work.
UPDATE: I'll drop recoding the display interface at the moment and only stick to the new pixel filter interface.