anchor: Shape:computeAABB
  Returns the points of the bounding box for the transformed shape.
  topLeftX, topLeftY, bottomRightX, bottomRightY = Shape:computeAABB( tx, ty, tr, childIndex )
  tx number The translation of the shape on the x-axis.
  ty number The translation of the shape on the y-axis.
  tr number The shape rotation.
  childIndex (1) number The index of the child to compute the bounding box of.
  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.