Don't initialize the audio buffer passed to the application's audio callback, since they are expected to entirely fill it with data or silence. For legacy apps that might expect the buffer to already have silence and thus may not fill the buffer in the callback, there's an environment variable to expose the old behaviour. Fixes Bugzilla #416.

The AmigaOS code has been removed from SDL, since it had been broken for a
 long time and had a few bits of fairly invasive code #ifdef'd into the
 SDL core.

However, there is an OS4 version of SDL here:
   http://www.rcdrummond.net/amiga/index.html

And a MorphOS version here:
   http://www.lehtoranta.net/powersdl/

--ryan.