Updates to SDL_sound

[Ryan C. Gordon]

> At the moment, aiff.c includes SDL_endian.h. That's no longer necessary,
> is it?

Sam seems to think that SDL_endian.h should NOT be included in SDL.h,
which doesn't make any sense to me, but oh well.

> Slight inconsistency: shn.c and voc.c use #if (defined
> SOUND_SUPPORTS_...). The other decoders use #ifdef SHOULD_SUPPORTS_...

I'll fix it.

> If I understand the last playsound.c changes, it now lets the decoder
> decide the audio format. Won't that break the decoders which can't or
> won't get the sample rate from their input, e.g. RAW, MOD, MIDI, ...?

(See last post.)

--ryan.