Transplanting SDL_audiocvt.c to SDL_sound

[Ryan C. Gordon]

> This is a first attempt at transplanting the audio converter from SDL
> to SDL_sound. I've reformatted the code and made some slight
> restructuring, but the only important change is that I've enabled the
> slow rate conversion filter.

It looks good, but I'd like to make an official release before we add the
audio conversion stuff to CVS, so I'll make that release tomorrow...I
_will_ put the midi part of the patch in there before the release, though.

Once I make this release, here's my thoughts on a roadmap for the NEXT
release after that. Some of these are easier than others...
 - Change the API (Ahhhhh!) to allow a decoder to register multiple file
   extensions. This is just some simple reworking.
 - Put a --credits option into playsound that lists SDL_sound contributors
   (you know who you are).
 - ADPCM compression support in wav.c (this covers most compressed .WAV
   files in existance).
 - Make aiff.c capable of handling multiple compression schemes (mostly,
   this involves updating it to match wav.c's changes, I think).
 - Fix mp3.c so it can handle decoding more than one MP3 in a row.
 - Start working on the audio conversion routines.
 - Manpages. I want manpages.
 - Officially declare this ported to MacOS X, if Max feels its ready.

That should be enough to move from 0.1.3 to 0.1.4.  :)

--ryan.