Add digger_math, some common useful math routines, inherited from
the rtpproxy project.
Add digger_math.
Reimplement FPS controlling mechanism around PLL ideas. This provides
much smoother control with less jitter. We are still limited by the
12.5 frames per second by default, the next step would be to decouple
game logic from that to provide faster screen update rate with the
same game speed.
Use SDL_DOUBLEBUF to significantly simplify update code.
Restore keyboard polling between levels when SDL is enabled. I am
not sure why I put that in there in the first place.