code: func (dc *Context) SetMask(mask *image.Alpha) error {
  	if mask.Bounds().Size() != dc.im.Bounds().Size() {
  		return errors.New("mask size must match context size")
  	}
  	dc.mask = mask
  	return nil
  }
docs: // SetMask allows you to directly set the *image.Alpha to be used as a clipping
  // mask. It must be the same size as the context, else an error is returned
  // and the mask is unchanged.