Blender Git Loki

VSE: Adding a panning angle for multichannel audio.

The panning angle allows a more intuitive panning when the output is
surround sound. It sets the angle on the horizontal plane around the
listener. 0 degrees is to the front, negative values go to the left and
positive ones to the right. +/-180 degrees is directly from the back.

Technical detail: the panning value is linear with the panning angle
with a factor of 90 degrees. For stereo this means that -1 is left and
+1 right, since the speakers are exactly 90 degrees to either side.

Differential Revision: https://developer.blender.org/D12275