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.