anchor: Fixture:getBoundingBox
  Returns the points of the fixture bounding box. In case the fixture has multiple children a 1-based index can be specified. For example, a fixture will have multiple children with a chain shape.
  topLeftX, topLeftY, bottomRightX, bottomRightY = Fixture:getBoundingBox( index )
  index (1) number A bounding box of the fixture.
  topLeftX number The x position of the top-left point.
  topLeftY number The y position of the top-left point.
  bottomRightX number The x position of the bottom-right point.
  bottomRightY number The y position of the bottom-right point.