First shot at new audio datatypes

…is in subversion for SDL 1.3. This is a work in progress, and I’d
appreciate if people could make sure it still compiles for them, etc.

There are still some gaps to fill in (SDL_LoadWAV() doesn’t understand
32-bit data yet, etc) and some bugs to fix (float32 converter isn’t
right yet), but you should be able to feed 32-bit data to an SDL 1.3
audio callback and have the backend convert it properly before feeding
it to the hardware, or push it through SDL_ConvertAudio() and get the
proper data out of it.