From the documentation, it is unclear to me whether GPAC should be able to DASH-prepare a video file with subtitles or not.

Using the `test5.mkv` sample file from the following URL: <https://github.com/ietf-wg-cellar/matroska-test-files/blob/master/test_files/test5.mkv>, I'm running GPAC like follows:

```bash
gpac -graph \
    'src=/tmp/test5.mkv' \
    'rfsrt:SID=#PID=5' @ 'tx3g2ttml:FID=subs' \
    'dst=/output/out.mpd:dmode=static:SID=#PID=0,#PID=1,subs'
```

I'm expecting GPAC to create a DASH stream with a manifest file at `/output/out.mpd`, that includes the Video stream (PID 0), the audio stream (PID 1), and the French subtitles (PID 5) in `ttml` format, each embed in their respective fragmented mp4 containers. But instead I'm getting:


```logs
[MP4Mux] Error creating new XML subtitle sample description: Bad Parameter
Failed to connect filter dasher PID text3 to filter mp4mx: Bad Parameter
[AVC|H264] xPS changed but could not flush frames before signaling state change !
session last connect error Bad Parameter

Filters connected:
ffdmx (src=/tmp/test5.mkv:gpac:src=/tmp/test5.mkv) (idx=1)
-(PID video1) ufnalu (dyn_idx=5)
--(PID video1) rfnalu (dyn_idx=6)
---(PID video1) dasher (dyn_idx=7)
----(PID manifest_mpd) fout (dst=/output/out.mpd:dmode=static:SID=#PID=0,#PID=1,subs) (idx=4)
----(PID video1)       mp4mx (dyn_idx=12)
-----(PID video1) fout (dst=/output/test5_dash_track1_init.mp4:gfopt:dmode=static:SID=#PID=0,#PID=1,subs:frag:xps_inband=no:psshs=moov:mime=video/mp4) (idx=10)
-(PID text3)  txtin (dyn_idx=8)
--(PID text3) tx3g2srt (dyn_idx=9)
---(PID text3) rfsrt (SID=#PID=5) (idx=2)
----(PID text3) tx3g2ttml (FID=subs) (idx=3)
-----(PID text3) dasher (dyn_idx=7)
mp4mx (dyn_idx=13)
Filters not connected:
fout (dst=/output/test5_dash_track5_init.mp4:gfopt:dmode=static:SID=#PID=0,#PID=1,subs:frag:xps_inband=no:psshs=moov:mime=application/mp4) (idx=11)
```

Apologies in advance if I missed something from the documentation.

---

GPAC build:

```
gpac - GPAC command line filter engine - version 2.3-DEV-rev64-g1659cd1a4-master
(c) 2000-2023 Telecom Paris distributed under LGPL v2.1+ - http://gpac.io
```

---

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 http://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: http://gpac.io/2013/07/16/how-to-file-a-bug-properly/

Can GPAC transform a pre-encoded video with embed subtitles to DASH format?
This is a GitHub Issue
repo:gpac
owner:gpac
Title : Can GPAC transform a pre-encoded video with embed subtitles to DASH format?
Issue date:

--- start body ---
From the documentation, it is unclear to me whether GPAC should be able to DASH-prepare a video file with subtitles or not.

Using the `test5.mkv` sample file from the following URL: <https://github.com/ietf-wg-cellar/matroska-test-files/blob/master/test_files/test5.mkv>, I'm running GPAC like follows:

```bash
gpac -graph \
    'src=/tmp/test5.mkv' \
    'rfsrt:SID=#PID=5' @ 'tx3g2ttml:FID=subs' \
    'dst=/output/out.mpd:dmode=static:SID=#PID=0,#PID=1,subs'
```

I'm expecting GPAC to create a DASH stream with a manifest file at `/output/out.mpd`, that includes the Video stream (PID 0), the audio stream (PID 1), and the French subtitles (PID 5) in `ttml` format, each embed in their respective fragmented mp4 containers. But instead I'm getting:


```logs
[MP4Mux] Error creating new XML subtitle sample description: Bad Parameter
Failed to connect filter dasher PID text3 to filter mp4mx: Bad Parameter
[AVC|H264] xPS changed but could not flush frames before signaling state change !
session last connect error Bad Parameter

Filters connected:
ffdmx (src=/tmp/test5.mkv:gpac:src=/tmp/test5.mkv) (idx=1)
-(PID video1) ufnalu (dyn_idx=5)
--(PID video1) rfnalu (dyn_idx=6)
---(PID video1) dasher (dyn_idx=7)
----(PID manifest_mpd) fout (dst=/output/out.mpd:dmode=static:SID=#PID=0,#PID=1,subs) (idx=4)
----(PID video1)       mp4mx (dyn_idx=12)
-----(PID video1) fout (dst=/output/test5_dash_track1_init.mp4:gfopt:dmode=static:SID=#PID=0,#PID=1,subs:frag:xps_inband=no:psshs=moov:mime=video/mp4) (idx=10)
-(PID text3)  txtin (dyn_idx=8)
--(PID text3) tx3g2srt (dyn_idx=9)
---(PID text3) rfsrt (SID=#PID=5) (idx=2)
----(PID text3) tx3g2ttml (FID=subs) (idx=3)
-----(PID text3) dasher (dyn_idx=7)
mp4mx (dyn_idx=13)
Filters not connected:
fout (dst=/output/test5_dash_track5_init.mp4:gfopt:dmode=static:SID=#PID=0,#PID=1,subs:frag:xps_inband=no:psshs=moov:mime=application/mp4) (idx=11)
```

Apologies in advance if I missed something from the documentation.

---

GPAC build:

```
gpac - GPAC command line filter engine - version 2.3-DEV-rev64-g1659cd1a4-master
(c) 2000-2023 Telecom Paris distributed under LGPL v2.1+ - http://gpac.io
```

---

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 http://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: http://gpac.io/2013/07/16/how-to-file-a-bug-properly/

--- end body ---
