more mpglib updates.

[Ryan C. Gordon]

I've made more mpglib patches. SDL_sound can now handle channels != 2, and
sample rates != 44100.

I've tried it on BeOS, and confirmed that mpglib works there, too.

Could someone with a Mac or some other bigendian CPU verify that mpglib
works there? decoders/mpglib.c currently expects the data it gets to be in
AUDIO_S16LSB format, but it might need to specify AUDIO_S16SYS (assuming
that mpglib itself is byteorder clean).

Assuming a PowerPC doesn't throw up on it, the only remaining problem with
mpglib is that it gives up and reports an error when it hits a corrupted
MP3 frame, and I'm willing to do without fixing that. SMPEG recovers
better, but mpglib can be a good option if you control all the assets you
plan on playing back.

(But if someone adds error recovery to mpglib, I'll take the patch.)

--ryan.