Lab和RGB互相转换代码，网上找了很多效果都不好，有的甚至转换过去再转换回来颜色都不一样了，有的存在除0的风险。改代码经过本人修改后验证的，用于图像处理效果都是对的。-Lab and RGB mutual conversion code, find a lot of the effect is not good, and some even converted in the past and back again are not the same color, there is some risk in addition to 0. Change the code after I modified validation for image processing effects are right.