Merge pull request #22248 from cudawarped:ffmpeg_rtsp_low_fps

Merge pull request #22248 from cudawarped:ffmpeg_rtsp_low_fps

* Allow the number of threads FFMpeg uses to be selected during VideoCapture::open().
Reset interupt timer in grab if
err = avformat_find_stream_info(ic, NULL);
is interupted but open is successful.

* Correct the returned number of threads and amend test cases.

* Update container test case.

* Reverse changes added to existing videoio_container test case and include test combining thread change and raw read in the newly added videoio_read test case.