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,
})  : assert(image != null),
      super(zIndex: zIndex, visible: visible);