Physics fixes

This fixes the 120Hz monitor issue (https://github.com/corpsmoderne/flappy-space-program/issues/1) and also a bug that made the force of a flap depend on the phyiscs framerate, plus minor cleanups.

Unfortunately the the fixed, low framerate causes some juddering now. Before, you probably just always got one physics frame per second on a 60Hz screen so it looked smooth. Lowering the physics timestep to e.g. 4ms fixes that, and really the physics are not very CPU intensive so that works well for me. To make it even smoother AND less CPU consuming though, linear interpolation could be added between the physics frames... I might just add that :)