Merge pull request #27864 from peters:patch-2

Add OPENCV_FFMPEG_SKIP_LOG_CALLBACK to preserve custom FFmpeg logging #27864 
 
OpenCV’s InternalFFMpegRegister overwrites av_log_set_callback, blocking custom FFmpeg log handlers in statically linked apps.

Add `OPENCV_FFMPEG_SKIP_LOG_CALLBACK` to let applications keep their own logging.

### 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/videoio/src/cap_ffmpeg_impl.hpp

@@ -937,6 +937,8 @@ static void ffmpeg_log_callback(void *ptr, int level, const char *fmt, va_list v
 
 class InternalFFMpegRegister
 {
+    static bool skip_log_callback;
+
 public:
     static void init(const bool threadSafe)
     {
@@ -949,6 +951,7 @@ public:
     {
         const bool debug_option = utils::getConfigurationParameterBool("OPENCV_FFMPEG_DEBUG");
         std::string level_option = utils::getConfigurationParameterString("OPENCV_FFMPEG_LOGLEVEL");
+        skip_log_callback = utils::getConfigurationParameterBool("OPENCV_FFMPEG_SKIP_LOG_CALLBACK");
         int level = AV_LOG_VERBOSE;
         if (!level_option.empty())
         {
@@ -957,7 +960,10 @@ public:
         if ( debug_option || (!level_option.empty()) )
         {
             av_log_set_level(level);
-            av_log_set_callback(ffmpeg_log_callback);
+            if (!skip_log_callback)
+            {
+                av_log_set_callback(ffmpeg_log_callback);
+            }
         }
         else
         {
@@ -986,10 +992,15 @@ public:
 #ifdef CV_FFMPEG_LOCKMGR
         av_lockmgr_register(NULL);
 #endif
-        av_log_set_callback(NULL);
+        if (!skip_log_callback)
+        {
+            av_log_set_callback(NULL);
+        }
     }
 };
 
+bool InternalFFMpegRegister::skip_log_callback = false;
+
 inline void fill_codec_context(AVCodecContext * enc, AVDictionary * dict)
 {
     if (!enc->thread_count)