Support more opponents

[NaN]

Profiling with 24 cars has revealed that we are spending the same amount of time in the sound as in physics. Sound is processed in a separate thread usually, thus not a big deal. I took the chance to cleanup the sampling code a bit though. So nothing exciting in this build. The new positions code will follow asap.

It should be possible to speed up physics too. A considerable amount of time is spent in wheel ray routines. We can reuse the wheel cylinder contact information to speed up them.

I've noticed that the game is not able to fully utilize the cpu(2/4cores). I am at max 50% per core load on the notebook(weak gpu) and desktop system(powerful gpu).

New Windows build(without data) vdrift-gui-2012-04-21.exe:
https://sourceforge.net/projects/vdrift/...%20builds/

Changes:
- Rewrite sound sampler to use integer math only instead of mixed floating point. This should allow the compiler to optimize it more aggressively.