This might be a little tricky to implement, but it's definitely
possible. The GLSL is actually pretty simple to learn once you look
into it. I recommend picking up a copy of the orange book. You will
basically need to be scaling and offsetting texcoord values, as well as
providing some sort of logic that defines which texture input is drawn
at each pixel. The tricky part will be explicitly setting the size of
each input texture and then the size of the output texture.