CENTERED AUTOPANNING

Auto-panning is only valid for stereo and mixing modes ``FULLPANNING''
or ``STEREOSURROUND''.

The volume levels left/right build the panning boundaries. The panning range
is the difference between left and right level. After enabling this you can
override the default levels with your own ones by using setVoiceVolume(..).
A default is provided to ensure sane initial settings. You can mute a voice
by setting left=right=0 or total=0.

Centered auto-panning starts in the middle, moves to one side and then moves
from left to right panning boundary (volume level) and vice versa.