audio: reduce glob_audio_state usage

audio: reduce glob_audio_state usage

Remove glob_audio_state from functions, where possible without breaking
the API.  This means that most static functions in audio.c now take an
AudioState pointer instead of implicitly using glob_audio_state.  Also
included a pointer in SWVoice*, HWVoice* structs, so that functions
dealing them can know the audio state without having to pass it around
separately.

This is required in order to support multiple simultaneous audio
backends (added in a later commit).

Signed-off-by: Kővágó, Zoltán <DirtY.iCE.hu@gmail.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-id: b5e241f24e795267b145bcde7c6a72dd5e6037ea.1566168923.git.DirtY.iCE.hu@gmail.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>