code: func (m Matrix) Unproject(u Vec) Vec {
  	det := m[0]*m[3] - m[2]*m[1]
  	return Vec{
  		(m[3]*(u.X-m[4]) - m[2]*(u.Y-m[5])) / det,
  		(-m[1]*(u.X-m[4]) + m[0]*(u.Y-m[5])) / det,
  	}
  }
docs: // Unproject does the inverse operation to Project.
  //
  // Time complexity is O(1).