Quaternion: Add missing call of _onChangeCallback in slerp().

src/math/Quaternion.js

@@ -562,7 +562,10 @@ Object.assign( Quaternion.prototype, {
 			this._y = s * y + t * this._y;
 			this._z = s * z + t * this._z;
 
-			return this.normalize();
+			this.normalize();
+			this._onChangeCallback();
+
+			return this;
 
 		}