code: func (g *GeoM) Skew(skewX, skewY float64) {
  	sx64 := math.Tan(skewX)
  	sy64 := math.Tan(skewY)
  	sx, sy := float32(sx64), float32(sy64)
  
  	a := (g.a_1 + 1) + g.c*sx
  	b := g.b + (g.d_1+1)*sx
  	c := (g.a_1+1)*sy + g.c
  	d := g.b*sy + (g.d_1 + 1)
  	tx := g.tx + g.ty*sx
  	ty := g.ty + g.tx*sy
  
  	g.a_1 = a - 1
  	g.b = b
  	g.c = c
  	g.d_1 = d - 1
  	g.tx = tx
  	g.ty = ty
  }
docs: // Skew skews the matrix by (skewX, skewY). The unit is radian.