code: func (c *ColorM) Apply(clr color.Color) color.Color {
  	return c.impl.Apply(clr)
  }
docs: // Apply pre-multiplies a vector (r, g, b, a, 1) by the matrix
  // where r, g, b, and a are clr's values in straight-alpha format.
  // In other words, Apply calculates ColorM * (r, g, b, a, 1)^T.