GTK and Windows appear to handle timers very differently:

GTK and Windows appear to handle timers very differently:
specifically, the elapsed time between calls varies much more with
GTK than it does under Windows. Therefore, I now take my own time
readings on every timer call, and this appears to have made the
animations run at closer to the same speed between platforms. Having
done that, I decided some of them were at the _wrong_ speed, and
fiddled with each game's timings as well.

[originally from svn r4189]