Implement SDL2 AUDIO_F32 format and use it as the default, since it has become the...

Implement SDL2 AUDIO_F32 format and use it as the default, since it has become the preferred format of more platforms these days, and SDL2 uses it internally.

Improved the fallback mechanism on SDL2 audio setup - by removing it; SDL2 will tell us what changes it makes for the format, and it will convert the rest automatically.

git-svn-id: svn://svn.icculus.org/twilight/trunk/darkplaces@12476 d7cf8633-e32d-0410-b094-e92efae38249