Fixed Sound Playing
sound_isplaying and audio_is_playing should not use a member variable on
sound_instance they should use the OAL function to test if the sound is
actually coming out the speaker to best emulate GM behaviour
Seperated Audio/Sound
Seperated the audio_* and sound_* functions into multiple files.
Fixed Typo
Fixed a typoe in previous commit.
Fixed Debug Mode
Polygonz was acessing the background_idmax in the defined macro before the
background struct header from universal systems was included, I also had to
define background_idmax in the background struct header.
Doxy Comments
Doxygen commented the General background header for graphics systems.
More Doxy
Generated more doxygen comments in the graphics system headers.
Merge pull request #139 from RobertBColton/master

Fixed Sound Playing