[SDL] callback issue

[Ryan C. Gordon]

> Call SDL_MixAudio(stream, (const Uint8*)data_p, len, SDL_MIX_MAXVOLUME)

Don't do that...currently the stream is always silence (so you don't 
have to mix against it, just memcpy), but since you're expected to 
completely fill the stream (with silence if nothing else), we're 
probably going to stop initializing the buffer before calling your 
callback...in which case you'd be mixing against random noise.

--ryan.