Thanks for reporting your issue. Please make sure these boxes are checked before submitting your issue - thank you!

- [x] I looked for a similar issue and couldn't find any.
- [x] I tried with the latest version of GPAC. Installers available at https://gpac.io/downloads/gpac-nightly-builds/
- [x] I give enough information for contributors to reproduce my issue (meaningful title, github labels, platform and compiler, command-line ...). I can share files anonymously with this dropbox: https://www.mediafire.com/filedrop/filedrop_hosted.php?drop=eec9e058a9486fe4e99c33021481d9e1826ca9dbc242a6cfaab0fe95da5e5d95

Detailed guidelines: https://gpac.io/bug-reporting/

I'm trying to transmit live subtitles from our transcoder to GPAC using Transport Stream.  Our transcoder does not support MP4 Text Stream, and from what I have read of GPAC's code, it does not support demuxing it either.

Instead of implementing the whole spec, which at the moment is overkill for my use case, I tried to mux SRT subtitles as private data in PES packets.

* The Elementary Stream uses PES packets private data with a `registration_descriptor` for the `SRT ` format.
![image](https://github.com/gpac/gpac/assets/2498576/e39f729a-17ef-42f1-a2bf-c7a8c11f8e85)
![image](https://github.com/gpac/gpac/assets/2498576/8fc725a8-9b5e-45f3-8cfd-1ef6cfda060c)

* Each PES packet contains one SRT event (aligned with PES start):

```
0
00:00:00,000 --> 00:00:01,000
This is the event zero

```

In GPAC, I managed to demux the stream in this branch: https://github.com/gpac/gpac/compare/master...jadarve:gpac:NS_ts_subtitles 

running a command like:

```
./bin/gcc/gpac \
  -i test_data/out_subtitle_only.ts \
  txtin \
  -o test_data/subtitles.vtt \
  -graph
```

prints the following graph:

```
Filters connected:
fin (src=test_data/out_subtitle_only.ts) (idx=1)
-(PID out_subtitle_only.ts) m2tsdmx (dyn_idx=4)
--(PID P1T2) writeuf (dyn_idx=5)
---(PID P1T2) writegen (dyn_idx=6)
----(PID P1T2) txtin (idx=2)
-----(PID P1T2) tx3g2vtt (dyn_idx=7)
------(PID P1T2) writegen (dyn_idx=8)
-------(PID P1T2) fout (dst=test_data/subtitles.vtt) (idx=3)
```

However, the generated WebVTT is not what I expected, it looks as:

```
WEBVTT

90:15:36.023 --> 90:15:39.623
1
01:00:00,000 --> 01:00:00,000
0
00:00:00,000 --> 00:00:00,040
current counter: 00000000000000000000000000000000000000000000000000
```

where I expected lots of subtitles lines.

This is just an example. My final goal is to be able to mux the TS subtitles into a CMAF track as WebVTT for live streaming. I would like to know if something is already done in that direction (live subtitle muxing), or if the approach I'm taking makes sense, and if you could advise me on how to proceed.
Support live subtitles stream from transport stream
This is a GitHub Issue
repo:gpac
owner:gpac
Title : Support live subtitles stream from transport stream
Issue date:

--- start body ---
Thanks for reporting your issue. Please make sure these boxes are checked before submitting your issue - thank you!

- [x] I looked for a similar issue and couldn't find any.
- [x] I tried with the latest version of GPAC. Installers available at https://gpac.io/downloads/gpac-nightly-builds/
- [x] I give enough information for contributors to reproduce my issue (meaningful title, github labels, platform and compiler, command-line ...). I can share files anonymously with this dropbox: https://www.mediafire.com/filedrop/filedrop_hosted.php?drop=eec9e058a9486fe4e99c33021481d9e1826ca9dbc242a6cfaab0fe95da5e5d95

Detailed guidelines: https://gpac.io/bug-reporting/

I'm trying to transmit live subtitles from our transcoder to GPAC using Transport Stream.  Our transcoder does not support MP4 Text Stream, and from what I have read of GPAC's code, it does not support demuxing it either.

Instead of implementing the whole spec, which at the moment is overkill for my use case, I tried to mux SRT subtitles as private data in PES packets.

* The Elementary Stream uses PES packets private data with a `registration_descriptor` for the `SRT ` format.
![image](https://github.com/gpac/gpac/assets/2498576/e39f729a-17ef-42f1-a2bf-c7a8c11f8e85)
![image](https://github.com/gpac/gpac/assets/2498576/8fc725a8-9b5e-45f3-8cfd-1ef6cfda060c)

* Each PES packet contains one SRT event (aligned with PES start):

```
0
00:00:00,000 --> 00:00:01,000
This is the event zero

```

In GPAC, I managed to demux the stream in this branch: https://github.com/gpac/gpac/compare/master...jadarve:gpac:NS_ts_subtitles 

running a command like:

```
./bin/gcc/gpac \
  -i test_data/out_subtitle_only.ts \
  txtin \
  -o test_data/subtitles.vtt \
  -graph
```

prints the following graph:

```
Filters connected:
fin (src=test_data/out_subtitle_only.ts) (idx=1)
-(PID out_subtitle_only.ts) m2tsdmx (dyn_idx=4)
--(PID P1T2) writeuf (dyn_idx=5)
---(PID P1T2) writegen (dyn_idx=6)
----(PID P1T2) txtin (idx=2)
-----(PID P1T2) tx3g2vtt (dyn_idx=7)
------(PID P1T2) writegen (dyn_idx=8)
-------(PID P1T2) fout (dst=test_data/subtitles.vtt) (idx=3)
```

However, the generated WebVTT is not what I expected, it looks as:

```
WEBVTT

90:15:36.023 --> 90:15:39.623
1
01:00:00,000 --> 01:00:00,000
0
00:00:00,000 --> 00:00:00,040
current counter: 00000000000000000000000000000000000000000000000000
```

where I expected lots of subtitles lines.

This is just an example. My final goal is to be able to mux the TS subtitles into a CMAF track as WebVTT for live streaming. I would like to know if something is already done in that direction (live subtitle muxing), or if the approach I'm taking makes sense, and if you could advise me on how to proceed.
--- end body ---
