[airstrike] bug in timing code

[Ulf Ekström]

> > The 2.6 kernel uses HZ=1000, so maybe this effect is gone.
> 
> I think it's 250Hz in latest kernel versions... :-)
> 
> I'm not sure it makes so much sense to tune the framerate according to a
> particular kernel with a particular scheduler (in newer kernels you can
> AFAIK pick which one you use).

I suggest we keep the 33 1/3 fps, because it really really helps on
those systems with a scheduler HZ of 100. On the more fine-grained
systems it should not matter a lot. The problem is that we really need
to keep a constant framerate for the motion to look smooth. In theory
we could interpolate between game frames, at least for the movement of
sprites (but not rotation in all cases), and run at whatever framerate
we get. But for the moment let's keep 30 ms/frame.

Ulf