png save error

修复:

保存图片的时候出现 颜色无法转换的问题

// CropImage by the specified box, quality is only for jpeg bin.
func CropImage(bin []byte, quality, x, y, width, height int) ([]byte, error) {
	img, typ, err := image.Decode(bytes.NewBuffer(bin))
	if err != nil {
		return nil, err
	}

	cropped := bytes.NewBuffer(nil)

	switch typ {
	case "png":
		switch img := img.(type) {
		case *image.NRGBA:
			img = img.SubImage(image.Rect(x, y, x+width, y+height)).(*image.NRGBA)
			err = png.Encode(cropped, img)
		case *image.RGBA:
			img = img.SubImage(image.Rect(x, y, x+width, y+height)).(*image.RGBA)
			err = png.Encode(cropped, img)
		default:
			return nil, fmt.Errorf("unsupported image type: %T", img)
		}

	case "jpeg":
		switch img := img.(type) {
		case *image.YCbCr:
			img = img.SubImage(image.Rect(x, y, x+width, y+height)).(*image.YCbCr)
			if quality == 0 {
				quality = 80
			}
			err = jpeg.Encode(cropped, img, &jpeg.Options{Quality: quality})
		default:
			return nil, fmt.Errorf("unsupported image type for jpeg: %T", img)
		}
	}

	return cropped.Bytes(), err
}

改进部分：增加色域匹配机制