code: func (m Matrix) Rotated(around Vec, angle float64) Matrix {
  	sint, cost := math.Sincos(angle)
  	m[4], m[5] = m[4]-around.X, m[5]-around.Y
  	m = m.Chained(Matrix{cost, sint, -sint, cost, 0, 0})
  	m[4], m[5] = m[4]+around.X, m[5]+around.Y
  	return m
  }
docs: // Rotated rotates everything around a given point by the given angle in radians.