Use box2d Coordinate System in libgdx

When using box2d in libgdx, pay attention to the following coordinate system problems:

1. the coordinates in the physical world are located at the center of the screen, while those in libgdx are located at the lower left corner.

2. The setviewport method is used to scale the stage, so that the drawing coordinates and the physical simulation coordinates are unified in form.

3. In actor that has been encapsulated by the body in box2d, the coordinates of the drawing should be calculated based on the coordinates and sizes of the body during draw, and then the drawing should be completed.

4. many Attributes in actor and texture are the same. In actor constructor, the coordinates and size values are preferentially used to set attributes of actor, then, relevant data is obtained through the actor's accessor method in subsequent use.