Merge pull request #27127 from sturkmen72:apng_has_hidden_frame

Changes about when APNG has a hidden frame #27127

closes : #27074

### 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
- [x] There is a reference to the original bug report and related work
- [x] 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

@@ -277,6 +277,8 @@ struct CV_EXPORTS_W_SIMPLE Animation
     CV_PROP_RW std::vector<int> durations;
     //! Vector of frames, where each Mat represents a single frame.
     CV_PROP_RW std::vector<Mat> frames;
+    //! image that can be used for the format in addition to the animation or if animation is not supported in the reader (like in PNG).
+    CV_PROP_RW Mat still_image;
 
     /** @brief Constructs an Animation object with optional loop count and background color.