Updated get_scrollRect in DisplayObject.hx

get_scrollRect should return a copy of the rectangle, to mimic the behavior on native flash platform.
On flash targets, the following is true:
    var s = new flash.display.Sprite();
    s.scrollRect = new flash.geom.Rectangle(0, 0, 20, 20);
    s.scrollRect.width = 40;
    trace(s.scrollRect.width);
    // outputs 20, not 40
    // only the copy of scrollRect was updated
    var rect = s.scrollRect;
    rect.width = 60;
    s.scrollRect = rect;
    trace(s.scrollRect.width);
    // outputs 60
    // the Sprite's scrollRect was updated