Merge pull request #25809 from zihaomu:imread_rgb_flag

imgcodecs: Add rgb flag for imread and imdecode #25809

Try to `imread` images by RGB to save R-B swapping costs.

## How to use it?
```
img_rgb = cv2.imread("PATH", IMREAD_COLOR_RGB) # OpenCV decode the image by RGB format.
```

## TODO
- [x] Fix the broken code
- [x] Add imread rgb test
- [x] Speed test of rgb mode.

## Performance test

| file name | IMREAD_COLOR  | IMREAD_COLOR_RGB |
| --------- | ------ | --------- |
| jpg01     | 284 ms | 277 ms    |
| jpg02     | 376 ms | 366 ms    |
| png01     | 62 ms  | 60 ms     |
| Png02     | 97 ms  | 94 ms     |

Test with [image_test.zip](https://github.com/user-attachments/files/15982949/image_test.zip)
```.cpp
string img_path = "/Users/mzh/work/data/image_test/png02.png";
int loop = 20;

TickMeter t;

double t0 = 10000;
for (int i = 0; i < loop; i++)
{
    t.reset();
    t.start();
    img_bgr = imread(img_path, IMREAD_COLOR);
    t.stop();

    if (t.getTimeMilli() < t0) t0 = t.getTimeMilli();
}

std::cout<<"bgr time = "<<t0<<std::endl;

t0 = 10000;
for (int i = 0; i < loop; i++)
{
    t.reset();
    t.start();
    img_rgb = imread(img_path, IMREAD_COLOR_RGB);
    t.stop();
    if (t.getTimeMilli() < t0) t0 = t.getTimeMilli();
}
std::cout<<"rgb time = "<<t0<<std::endl;
``` 
### Pull Request Readiness Checklist

See details at https://github.com/opencv/opencv/wiki/How_to_contribute#making-a-good-pull-request

- [x] I agree to contribute to the project under Apache 2 License.
- [x] To the best of my knowledge, the proposed patch is not based on a code under GPL or another license that is incompatible with OpenCV
- [x] The PR is proposed to the proper branch
- [ ] There is a reference to the original bug report and related work
- [ ] There is accuracy test, performance test and test data in opencv_extra repository, if applicable
      Patch to opencv_extra has the same branch name.
- [ ] The feature is well documented and sample code can be built with the project CMake

modules/imgcodecs/include/opencv2/imgcodecs.hpp

@@ -68,7 +68,8 @@ namespace cv
 enum ImreadModes {
        IMREAD_UNCHANGED            = -1, //!< If set, return the loaded image as is (with alpha channel, otherwise it gets cropped). Ignore EXIF orientation.
        IMREAD_GRAYSCALE            = 0,  //!< If set, always convert image to the single channel grayscale image (codec internal conversion).
-       IMREAD_COLOR                = 1,  //!< If set, always convert image to the 3 channel BGR color image.
+       IMREAD_COLOR_BGR            = 1,  //!< If set, always convert image to the 3 channel BGR color image.
+       IMREAD_COLOR                = 1,  //!< Same as IMREAD_COLOR_BGR.
        IMREAD_ANYDEPTH             = 2,  //!< If set, return 16-bit/32-bit image when the input has the corresponding depth, otherwise convert it to 8-bit.
        IMREAD_ANYCOLOR             = 4,  //!< If set, the image is read in any possible color format.
        IMREAD_LOAD_GDAL            = 8,  //!< If set, use the gdal driver for loading the image.
@@ -78,7 +79,8 @@ enum ImreadModes {
        IMREAD_REDUCED_COLOR_4      = 33, //!< If set, always convert image to the 3 channel BGR color image and the image size reduced 1/4.
        IMREAD_REDUCED_GRAYSCALE_8  = 64, //!< If set, always convert image to the single channel grayscale image and the image size reduced 1/8.
        IMREAD_REDUCED_COLOR_8      = 65, //!< If set, always convert image to the 3 channel BGR color image and the image size reduced 1/8.
-       IMREAD_IGNORE_ORIENTATION   = 128 //!< If set, do not rotate the image according to EXIF's orientation flag.
+       IMREAD_IGNORE_ORIENTATION   = 128, //!< If set, do not rotate the image according to EXIF's orientation flag.
+       IMREAD_COLOR_RGB            = 256, //!< If set, always convert image to the 3 channel RGB color image.
      };
 
 //! Imwrite flags
@@ -268,7 +270,7 @@ Currently, the following file formats are supported:
 @param filename Name of file to be loaded.
 @param flags Flag that can take values of cv::ImreadModes
 */
-CV_EXPORTS_W Mat imread( const String& filename, int flags = IMREAD_COLOR );
+CV_EXPORTS_W Mat imread( const String& filename, int flags = IMREAD_COLOR_BGR );
 
 /** @brief Loads an image from a file.
 
@@ -279,7 +281,7 @@ This is an overloaded member function, provided for convenience. It differs from
 @note
 The image passing through the img parameter can be pre-allocated. The memory is reused if the shape and the type match with the load image.
  */
-CV_EXPORTS_W void imread( const String& filename, OutputArray dst, int flags = IMREAD_COLOR );
+CV_EXPORTS_W void imread( const String& filename, OutputArray dst, int flags = IMREAD_COLOR_BGR );
 
 /** @brief Loads a multi-page image from a file.