code: func (f *Fpdf) parsepng(r io.Reader, readdpi bool) (info *ImageInfoType) {
  	buf, err := bufferFromReader(r)
  	if err != nil {
  		f.err = err
  		return
  	}
  	return f.parsepngstream(buf, readdpi)
  }
docs: // parsepng extracts info from a PNG data