draggable start callback returns undefined ui.offset and ui.position

[DRHansen]

this.position and this.positionAbs are not set before calling this._trigger("start", event) in _mouseStart:

If they are set by adding:

  // 2009-03-20 14:14:12 DRH
  //Compute the helpers position
  this.position = this._generatePosition(event);
  this.positionAbs = this._convertPositionTo("absolute");

just before:

  //Call plugins and callbacks
  this._trigger("start", event);

proper values are available for _uiHash() when it creates the ui object in _trigger()

[paul]

Fixed in #3106.