anchor: EdgeShape:getPreviousVertex
  Gets the vertex that establishes a connection to the previous shape.
  
  Setting next and previous EdgeShape vertices can help prevent unwanted collisions when a flat shape slides along the edge and moves over to the new shape.
  x, y = EdgeShape:getPreviousVertex()
  x number The x-component of the vertex, or nil if EdgeShape:setPreviousVertex hasn't been called.
  y number The y-component of the vertex, or nil if EdgeShape:setPreviousVertex hasn't been called.