error: ‘CV_LOAD_IMAGE_UNCHANGED’ was not declared in this scope

1-错误
2-错误原因
opencv4.x以上，有些宏，API名字改了，需要改为新的
3-解决方案

 CV_LOAD_IMAGE_UNCHANGED 改为 cv::IMREAD_UNCHANGED
   CV_LOAD_IMAGE_GRAYSCALE  改为 cv::IMREAD_GRAYSCALE
   CV_LOAD_IMAGE_COLOR    改为 cv::IMREAD_COLOR
   CV_LOAD_IMAGE_ANYDEPTH   改为 cv::IMREAD_ANYDEPTH