The reference volume is to be used as reference volume for stored stream
volumes. Previously if a new stream was created the relative volume was
taken relatively to the virtual device volume. Due to the flat volume
logic this could then be fed back to the virtual device volume.
Repeating the whole story over and over would result in a device volume
that would go lower, and lower and lower.
This patch introduces a 'reference' volume for each sink which stays
unmodified by stream volume changes even if flat volumes are used. It is
only modified if the sink volumes are modified directly by the user.
For further explanations see http://pulseaudio.org/wiki/InternalVolumes