- use the overall volume managed by the playlist rather than
     aout_volumeGet() that now only works when a aout is running.
     This is no problem since the skins2 volume slider functionally
     reflects this overall volume managed by the playlist anyway.
   - stick to what include/vlc_aout_intf.h lets us know about volume
     boundaries(as of today, AOUT_VOLUME_DEFAULT and AOUT_VOLUME_MAX)
     and infer volume to be in the [0., AOUT_VOLUME_MAX/AOUT_VOLUME_DEFAULT]
     range as requested by some functions taking a float as parameter.