code: func (i *Image) makeStale() {
  	i.basePixels = nil
  	i.drawTrianglesHistory = nil
  	i.stale = true
  
  	// Don't have to call makeStale recursively here.
  	// Restoring is done after topological sorting is done.
  	// If an image depends on another stale image, this means that
  	// the former image can be restored from the latest state of the latter image.
  }
docs: // makeStale makes the image stale.