message: allow forcing the input format for ffmpeg backend with env var
diff: @@ -891,7 +891,14 @@ bool CvCapture_FFMPEG::open( const char* _filename )
  #else
  av_dict_set(&dict, "rtsp_transport", "tcp", 0);
  #endif
  - int err = avformat_open_input(&ic, _filename, NULL, &dict);
  + AVInputFormat* input_format = NULL;
  + AVDictionaryEntry* entry = av_dict_get(dict, "input_format", NULL, 0);
  + if (entry != 0)
  + {
  + input_format = av_find_input_format(entry->value);
  + }
  +
  + int err = avformat_open_input(&ic, _filename, input_format, &dict);
  #else
  int err = av_open_input_file(&ic, _filename, NULL, 0, NULL);
  #endif
  