code: func (g *GeoM) Invert() {
  	det := g.det()
  	if det == 0 {
  		panic("ebiten: g is not invertible")
  	}
  
  	a := (g.d_1 + 1) / det
  	b := -g.b / det
  	c := -g.c / det
  	d := (g.a_1 + 1) / det
  	tx := (-(g.d_1+1)*g.tx + g.b*g.ty) / det
  	ty := (g.c*g.tx + -(g.a_1+1)*g.ty) / det
  
  	g.a_1 = a - 1
  	g.b = b
  	g.c = c
  	g.d_1 = d - 1
  	g.tx = tx
  	g.ty = ty
  }
docs: // Invert inverts the matrix.
  // If g is not invertible, Invert panics.