anchor: Shape:computeMass
  Computes the mass properties for the shape with the specified density.
  x, y, mass, inertia = Shape:computeMass( density )
  density number The shape density.
  x number The x postition of the center of mass.
  y number The y postition of the center of mass.
  mass number The mass of the shape.
  inertia number The rotational inertia.