// Add buffering if r does not provide ReadByte.
	if rr, ok := r.(reader); ok {
		d.r = rr
	} else {
		d.r = bufio.NewReader(r)
	}

	d.loopCount = -1

	err := d.readHeaderAndScreenDescriptor()
	if err != nil {
		return err
	}
	if configOnly {
		return nil
	}

	for {
		c, err := readByte(d.r)
		if err != nil {
			return fmt.Errorf("gif: reading frames: %v", err)