Better handling of ifdefs for audio

* So it can work reliably in all cases (all vga modes with or without
audio)
beamracer: adjust for 320x240 video mode.

Works again, but the rotation and alitudes need tweaking. Currently it
looks a bit HYPERSPACE!
Optimize a bit rendering of the car sprite

* Trim useless borders from the sprite and clean it up
* Use magic pink rather than white as the transparent color
* Make sure the sprite is converted to RGB in mk_sprite (so paletized
images work)
* Adjust coordinates as the trimmed sprite is smaller
Remove beam reacer, moved to a separate project.