Merge pull request #28844 from 0lekW:videoio-image-seq-start

Add CAP_PROP_IMAGE_SEQ_START #28844

Resolves https://github.com/opencv/opencv/issues/28843

New CAP_PROP_IMAGE_SEQ_START as an open only property for setting the first frame index when a VideoCap is opening with pattern, squashes test_images TODO.

`CAP_FFMPEG`: forwarded to the image2 demuxers "start_number" option.

`CAP_IMAGES`: Now a WithParams variant so the value reaches open() before probe runs, replaces 0 or 1 auto probe for sequence start.

Property is opt in so default behavior is not affected.  
Please note that G_STREAMER is not considered here as I think its out of scope, it already has a "start-index" property and its backend is driven by the pipeline strings rather then these printf patterns.

If an alternative solution is preferred that will not involve a new property or promoting CAP_IMAGES to a "WithParams" variant I don't mind implementing.  

### 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.
- [x] The feature is well documented and sample code can be built with the project CMake

modules/videoio/src/cap_ffmpeg_impl.hpp

@@ -624,6 +624,7 @@ struct CvCapture_FFMPEG
     int use_opencl;
     int extraDataIdx;
     int requestedThreads;
+    int image_seq_start;  // image2 demuxer start_number; -1 means unset
 };
 
 void CvCapture_FFMPEG::init()
@@ -679,6 +680,7 @@ void CvCapture_FFMPEG::init()
     use_opencl = 0;
     extraDataIdx = 1;
     requestedThreads = cv::getNumberOfCPUs();
+    image_seq_start = -1;
 }
 
 
@@ -1160,6 +1162,10 @@ bool CvCapture_FFMPEG::open(const char* _filename, int index, const Ptr<IStreamR
         {
             nThreads = requestedThreads = params.get<int>(CAP_PROP_N_THREADS);
         }
+        if (params.has(CAP_PROP_IMAGE_SEQ_START))
+        {
+            image_seq_start = params.get<int>(CAP_PROP_IMAGE_SEQ_START);
+        }
         if (params.warnUnusedParameters())
         {
             CV_LOG_ERROR(NULL, "VIDEOIO/FFMPEG: unsupported parameters in .open(), see logger INFO channel for details. Bailout");
@@ -1194,6 +1200,10 @@ bool CvCapture_FFMPEG::open(const char* _filename, int index, const Ptr<IStreamR
         av_dict_set(&dict, "rtsp_transport", "tcp", 0);
 #endif
     }
+    if (image_seq_start >= 0)
+    {
+        av_dict_set_int(&dict, "start_number", image_seq_start, 0);
+    }
     CV_FFMPEG_FMT_CONST AVInputFormat* input_format = NULL;
     AVDictionaryEntry* entry = av_dict_get(dict, "input_format", NULL, 0);
     if (entry != 0)