Implement scroll wheel zooming for using GLUT

So, on some thinkpads (with the little nub mouse), the middle mousebutton isn't always emulated when you press both left and right buttons (which is what happens on most other laptops).

Without a middle mouse button, the only other way to fly in and out is to use the scroll wheel events (which thinkpads _do_ produce).

This PR enables fly in/out on thinkpads, and scroll wheel fly in/out for normal mice.

Thanks,
Hurricane