- [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 

I'm trying to encapsulate a TTML chunk into an fMP4 container.
Related: #2730 

Back then, when trying to do the same with WebVTT, I was finally succesful thanks to your guidance.
So I tried the same command but using TTML input, and the output seems to have some problem.

This is an example input:
```xml
<?xml version="1.0" encoding="UTF-8"?>
<tt xmlns="http://www.w3.org/ns/ttml" xmlns:ttm="http://www.w3.org/ns/ttml#metadata" xmlns:ttp="http://www.w3.org/ns/ttml#parameter" xmlns:tts="http://www.w3.org/ns/ttml#styling" xml:lang="en" >
  <head>
    <metadata>
      <ttm:title/>
      <ttm:desc/>
      <ttm:copyright/>
    </metadata>
    <styling>
      <style xml:id="style0" tts:textAlign="center" />
    </styling>
    <layout>
      <region xml:id="region01" tts:extent="100% 14%" tts:fontFamily="proportionalSansSerif" tts:fontSize="24px" tts:origin="0% 79%" />
    </layout>
  </head>
  <body>
    <div>
      <p style="style0" end="06:48:23.888" region="region01" begin="06:48:20.051" xml:id="caption0152" >Abrazándose. No robándose las perlas<br/>de la abuela Fine.</p>
    </div>
  </body>
</tt>
```

Then I run this gpac command:
```
# gpac -i input.ttml -o output.mpd:dmode=static:segdur=6.4:dur=6.4:noinit=false:segext=m4s:initext=mp4:timescale=1000:force_flush=true:moovts=1000:subs_sidx=0:store=fstart:mediats=1000
```

And this is what the output contains:
```
$ cat input_dash1.m4s
stypmsdhmsdhmsixlmsg,sidx��u�P�hmoofmfhdPtraftfhd&tfdt$trunpu�PTmdat<?xml version="1.0" encoding="UTF-8"?>
<tt xmlns="http://www.w3.org/ns/ttml" xmlns:ttm="http://www.w3.org/ns/ttml#metadata" xmlns:ttp="http://www.w3.org/ns/ttml#parameter" xmlns:tts="http://www.w3.org/ns/ttml#styling" xml:lang="en" >
  <head>
    <metadata>
      <ttm:title/>
      <ttm:desc/>
      <ttm:copyright/>
    </metadata>
    <styling>
      <style xml:id="style0" tts:textAlign="center" />
    </styling>
    <layout>
      <region xml:id="region01" tts:extent="100% 14%" tts:fontFamily="proportionalSansSerif" tts:fontSize="24px" tts:origin="0% 79%" />
    </layout>
  </head>
  <body>
    <div>
      <p style="style0" end="06:48:23.888" region="region01" begin="06:48:20.051" xml:id="caption0152" >Abrazándose. No robándose las perlas<br/>de la abuela Fine.</p>
    </div>
  </body>
</tt><?xml version="1.0" encoding="UTF-8"?>
<tt xmlns="http://www.w3.org/ns/ttml" xmlns:ttm="http://www.w3.org/ns/ttml#metadata" xmlns:ttp="http://www.w3.org/ns/ttml#parameter" xmlns:tts="http://www.w3.org/ns/ttml#styling" xml:lang="en" >
  <head>
    <metadata>
      <ttm:title/>
      <ttm:desc/>
      <ttm:copyright/>
    </metadata>
    <styling>
      <style xml:id="style0" tts:textAlign="center" />
    </styling>
    <layout>
      <region xml:id="region01" tts:extent="100% 14%" tts:fontFamily="proportionalSansSerif" tts:fontSize="24px" tts:origin="0% 79%" />
    </layout>
  </head>
  <body>
    <div>
      <p style="style0" end="06:48:23.888" region="region01" begin="06:48:20.051" xml:id="caption0152" >Abrazándose. No robándose las perlas<br/>de la abuela Fine.</p>
    </div>
  </body>
</tt>
```

As you can see, the xml content is duplicated inside the fMP4 container.
I didn't get playback on players, and that's how I end up checking inside the file. Didn't found a workaround with my known gpac commands, and this seems like some kind of error, so I create this issue here.

Tested with a mainline build minutes ago:
```
# gpac --asdasd
Nothing to do, check usage "gpac -h"
gpac - GPAC command line filter engine - version 2.3-DEV-rev922-g96b474ec5-master
(c) 2000-2023 Telecom Paris distributed under LGPL v2.1+ - https://gpac.io

Please cite our work in your research:
	GPAC Filters: https://doi.org/10.1145/3339825.3394929
	GPAC: https://doi.org/10.1145/1291233.1291452
```

Thanks.
Repeated ttml code when encapsulating inside fMP4
This is a GitHub Issue
repo:gpac
owner:gpac
Title : Repeated ttml code when encapsulating inside fMP4
Issue date:

--- start body ---
- [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 

I'm trying to encapsulate a TTML chunk into an fMP4 container.
Related: #2730 

Back then, when trying to do the same with WebVTT, I was finally succesful thanks to your guidance.
So I tried the same command but using TTML input, and the output seems to have some problem.

This is an example input:
```xml
<?xml version="1.0" encoding="UTF-8"?>
<tt xmlns="http://www.w3.org/ns/ttml" xmlns:ttm="http://www.w3.org/ns/ttml#metadata" xmlns:ttp="http://www.w3.org/ns/ttml#parameter" xmlns:tts="http://www.w3.org/ns/ttml#styling" xml:lang="en" >
  <head>
    <metadata>
      <ttm:title/>
      <ttm:desc/>
      <ttm:copyright/>
    </metadata>
    <styling>
      <style xml:id="style0" tts:textAlign="center" />
    </styling>
    <layout>
      <region xml:id="region01" tts:extent="100% 14%" tts:fontFamily="proportionalSansSerif" tts:fontSize="24px" tts:origin="0% 79%" />
    </layout>
  </head>
  <body>
    <div>
      <p style="style0" end="06:48:23.888" region="region01" begin="06:48:20.051" xml:id="caption0152" >Abrazándose. No robándose las perlas<br/>de la abuela Fine.</p>
    </div>
  </body>
</tt>
```

Then I run this gpac command:
```
# gpac -i input.ttml -o output.mpd:dmode=static:segdur=6.4:dur=6.4:noinit=false:segext=m4s:initext=mp4:timescale=1000:force_flush=true:moovts=1000:subs_sidx=0:store=fstart:mediats=1000
```

And this is what the output contains:
```
$ cat input_dash1.m4s
stypmsdhmsdhmsixlmsg,sidx��u�P�hmoofmfhdPtraftfhd&tfdt$trunpu�PTmdat<?xml version="1.0" encoding="UTF-8"?>
<tt xmlns="http://www.w3.org/ns/ttml" xmlns:ttm="http://www.w3.org/ns/ttml#metadata" xmlns:ttp="http://www.w3.org/ns/ttml#parameter" xmlns:tts="http://www.w3.org/ns/ttml#styling" xml:lang="en" >
  <head>
    <metadata>
      <ttm:title/>
      <ttm:desc/>
      <ttm:copyright/>
    </metadata>
    <styling>
      <style xml:id="style0" tts:textAlign="center" />
    </styling>
    <layout>
      <region xml:id="region01" tts:extent="100% 14%" tts:fontFamily="proportionalSansSerif" tts:fontSize="24px" tts:origin="0% 79%" />
    </layout>
  </head>
  <body>
    <div>
      <p style="style0" end="06:48:23.888" region="region01" begin="06:48:20.051" xml:id="caption0152" >Abrazándose. No robándose las perlas<br/>de la abuela Fine.</p>
    </div>
  </body>
</tt><?xml version="1.0" encoding="UTF-8"?>
<tt xmlns="http://www.w3.org/ns/ttml" xmlns:ttm="http://www.w3.org/ns/ttml#metadata" xmlns:ttp="http://www.w3.org/ns/ttml#parameter" xmlns:tts="http://www.w3.org/ns/ttml#styling" xml:lang="en" >
  <head>
    <metadata>
      <ttm:title/>
      <ttm:desc/>
      <ttm:copyright/>
    </metadata>
    <styling>
      <style xml:id="style0" tts:textAlign="center" />
    </styling>
    <layout>
      <region xml:id="region01" tts:extent="100% 14%" tts:fontFamily="proportionalSansSerif" tts:fontSize="24px" tts:origin="0% 79%" />
    </layout>
  </head>
  <body>
    <div>
      <p style="style0" end="06:48:23.888" region="region01" begin="06:48:20.051" xml:id="caption0152" >Abrazándose. No robándose las perlas<br/>de la abuela Fine.</p>
    </div>
  </body>
</tt>
```

As you can see, the xml content is duplicated inside the fMP4 container.
I didn't get playback on players, and that's how I end up checking inside the file. Didn't found a workaround with my known gpac commands, and this seems like some kind of error, so I create this issue here.

Tested with a mainline build minutes ago:
```
# gpac --asdasd
Nothing to do, check usage "gpac -h"
gpac - GPAC command line filter engine - version 2.3-DEV-rev922-g96b474ec5-master
(c) 2000-2023 Telecom Paris distributed under LGPL v2.1+ - https://gpac.io

Please cite our work in your research:
	GPAC Filters: https://doi.org/10.1145/3339825.3394929
	GPAC: https://doi.org/10.1145/1291233.1291452
```

Thanks.
--- end body ---
