[SDL] multiple audio devices...

[Ryan C. Gordon]

Some initial work is done on multiple audio devices for SDL 1.3.

This is only working on Mac OS X at the moment (with 9/10ths of the OSS 
work done for Linux), but those that want to check it out and comment 
can pull this Subversion branch:

     svn://libsdl.org/branches/SDL-ryan-multiple-audio-device/

The audio drivers that haven't been updated won't compile, which is most 
of them at the moment.

Take a look at test/testaudioinfo.c for how to enumerate audio devices, 
and test/testmultiaudio.c for a rundown of how to select a device for 
playback (or select several to play from multiple sound cards at once).

Comments are welcome; the API is definitely still open to changes at 
this point, and the internals are still changing under the hood anyhow. 
Even when it's solid, there're about 25 (!) audio backends to update, 
which is crazy.

--ryan.