PositionalAudio: Use HRTF panninModel for better results.

src/audio/PositionalAudio.js

@@ -12,6 +12,7 @@ function PositionalAudio( listener ) {
 	Audio.call( this, listener );
 
 	this.panner = this.context.createPanner();
+	this.panningModel = 'HRTF';
 	this.panner.connect( this.gain );
 
 }