A special value for the pan property of samples and audio streams.
Use this value to disable panning on samples and audio streams, and play
them without attentuation implied by panning support.
ALLEGRO_AUDIO_PAN_NONE is different from a pan value of 0.0 (centered)
because, when panning is enabled, we try to maintain a constant sound
power level as a sample is panned from left to right.
A sound coming out of one speaker should sound as loud as it does when
split over two speakers.
As a consequence, a sample with pan value 0.0 will be 3 dB softer than
the original level.
(Please correct us if this is wrong.)