Merge pull request #26854 from vrabaud:png_leak

Fix oss-fuzz bugs 391934081 and 392318892 #26854

- fix a potential overflow in x0+w0
- use the proper function to deal with background color to deal with all cases of the spec
- use BGR layout for APNG background color

### 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/include/opencv2/imgcodecs.hpp

@@ -263,7 +263,7 @@ struct CV_EXPORTS_W_SIMPLE Animation
     - If a negative value or a value beyond the maximum of `0xffff` (65535) is provided, it is reset to `0`
     (infinite looping) to maintain valid bounds.
 
-    @param bgColor A `Scalar` object representing the background color in BGRA format:
+    @param bgColor A `Scalar` object representing the background color in BGR format:
     - Defaults to `Scalar()`, indicating an empty color (usually transparent if supported).
     - This background color provides a solid fill behind frames that have transparency, ensuring a consistent display appearance.
     */