Regression analysis on `bgc.sub.dub.ogg`

[softworkz]

reported by Michael Niedermayer on https://lists.ffmpeg.org/mailman/listinfo/ffmpeg-devel/:
(ffstaging/FFmpeg#18)

Causes this testcase to fail:
./ffmpeg -i 'bgc.sub.dub.ogm' -vframes 3 -bitexact -y nosubs.webm

https://samples.ffmpeg.org/ogg/

I did not investgate why or if this a bug or expected. Just reporting a difference
ive seen

[softworkz]

I have run this command with the Subtitle Filtering patchset applied => nosubs.webm

And with a downloaded binary (n5.1-1-g6d83c5f876-20220730) => nosubs2.webm

Now we can compare the three files (original and the two outputs):

The original file has 496 frames in 4 streams:

The file nosubs2.webm (output from stock ffmpeg) has only 2 frames, one video and one audio frame:

The file nosubs.webm (created from ffmpeg with the subtitle patchset applied) has 

• 52 audio frames 
  followed by
• 3 video frames

No more frames appear after the three video frames.

Considering the -vframes 3 parameter, this would appear to me as a proper result, while the output of the stock ffmpeg does not.