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/perf/perf_jpeg.cpp

@@ -27,6 +27,23 @@ PERF_TEST(JPEG, Decode)
     SANITY_CHECK_NOTHING();
 }
 
+PERF_TEST(JPEG, Decode_rgb)
+{
+    String filename = getDataPath("stitching/boat1.jpg");
+
+    FILE *f = fopen(filename.c_str(), "rb");
+    fseek(f, 0, SEEK_END);
+    long len = ftell(f);
+    fseek(f, 0, SEEK_SET);
+    vector<uchar> file_buf((size_t)len);
+    EXPECT_EQ(len, (long)fread(&file_buf[0], 1, (size_t)len, f));
+    fclose(f); f = NULL;
+
+    TEST_CYCLE() imdecode(file_buf, IMREAD_COLOR_RGB);
+
+    SANITY_CHECK_NOTHING();
+}
+
 PERF_TEST(JPEG, Encode)
 {
     String filename = getDataPath("stitching/boat1.jpg");