BMFGround constructor
- {@required String image,
 - double width,
 - double height,
 - double anchorX,
 - double anchorY,
 - int zoomLevel,
 - BMFCoordinateBounds bounds,
 - BMFCoordinate position,
 - double transparency: 1.0,
 - int zIndex: 0,
 - bool visible: true}
 
BMFGround构造方法
Implementation
BMFGround(
    {@required this.image,
    this.width,
    this.height,
    this.anchorX,
    this.anchorY,
    this.zoomLevel,
    this.bounds,
    this.position,
    this.transparency: 1.0,
    int zIndex: 0,
    bool visible: true})
    : super(zIndex: zIndex, visible: visible);