Merge pull request #26849 from sturkmen72:apng-writeanimation

APNG encoding optimization #26849

related #26840

### 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
- [ ] 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/test/test_common.hpp

@@ -9,6 +9,7 @@ namespace opencv_test {
 
 Mat generateTestImageBGR();
 Mat generateTestImageGrayscale();
+void readFileBytes(const std::string& fname, std::vector<unsigned char>& buf);
 
 }  // namespace