Normalizing the AudioListener front orientation vectors before taking their
cross product avoids the possibility of overflow.
The panning effect and the azimuth and elevation calculation in the Web Audio
spec becomes undefined with linearly dependent listener vectors, so keep
existing state in these situations.
PannerNode orientation is normalized for consistency, but zero is permitted
for this vector because the sound cone algorithm in the Web Audio specifies
behavior for this case.
--HG--
extra : transplant_source : %DA%C7e%E7%90%14%AF%EA%08%94x%C1%A2g%F1%9A%EE%16%EB%29
