anchor: PulleyJoint:getGroundAnchors
  Get the ground anchor positions in world coordinates.
  a1x, a1y, a2x, a2y = PulleyJoint:getGroundAnchors()
  a1x number The x coordinate of the first anchor.
  a1y number The y coordinate of the first anchor.
  a2x number The x coordinate of the second anchor.
  a2y number The y coordinate of the second anchor.