code: func opaque(m image.Image) bool {
  	if o, ok := m.(opaquer); ok {
  		return o.Opaque()
  	}
  	b := m.Bounds()
  	for y := b.Min.Y; y < b.Max.Y; y++ {
  		for x := b.Min.X; x < b.Max.X; x++ {
  			_, _, _, a := m.At(x, y).RGBA()
  			if a != 0xffff {
  				return false
  			}
  		}
  	}
  	return true
  }
docs: // Returns whether or not the image is fully opaque.