Hi,

Thanks in advance for any help. Please let me know if I should send more information privately.

**Summary**
I had a `sample.mp4` file which has embedded EMSG boxes carrying custom metadata. To serve it via DASH, I used MP4Box to generate the mpd manifest file, and MP4Box generates two files for me

- `sample_dash.mpd`
- `sample_dashinit.mp4`

**Expected Result**
The EMSG boxes in the original fMP4 is preserved in `sample_dashinit.mp4` and served via `sample_dash.mpd`

**Actual Result**
The generated sample_dashinit.mp4 no longer have EMSG boxes.

**Repro Steps**

1. Take any fragmented MP4 file with EMSG boxes
2. run `MP4Box -dash 100000 sample.mp4`
3. Load the generated `sample_dashinit.mp4` using https://gpac.github.io/mp4box.js/test/filereader.html, and no EMSG boxes are in BoxView > TreeView

I might have missed any usage to preserve the EMSG boxes - can someone help advice any proper usage of MP4Box or any workaround in this case? Thanks.

Also copying the generated mpd file
```
<?xml version="1.0"?>
<!-- MPD file Generated with GPAC version 1.1.0-DEV-rev1759-geb2d1e6dd-master at 2022-02-17T06:23:14.473Z -->
<MPD xmlns="urn:mpeg:dash:schema:mpd:2011" minBufferTime="PT1.500S" type="static" mediaPresentationDuration="PT0H0M23.992S" maxSegmentDuration="PT0H0M23.993S" profiles="urn:mpeg:dash:profile:full:2011">
 <ProgramInformation moreInformationURL="http://gpac.io">
  <Title>sample_dash.mpd generated by GPAC</Title>
 </ProgramInformation>

 <Period duration="PT0H0M23.992S">
  <AdaptationSet segmentAlignment="true" maxWidth="1920" maxHeight="1080" maxFrameRate="96000/3199" par="16:9" lang="und" startWithSAP="1">
   <InbandEventStream schemeIdUri="urn:mpeg:dash:event:2012" value="1"/>
   <Representation id="1" mimeType="video/mp4" codecs="hev1.1.6.L150.0" width="1920" height="1080" frameRate="96000/3199" sar="1:1" bandwidth="3030916">
    <BaseURL>sample_dashinit.mp4</BaseURL>
    <SegmentList timescale="96000" duration="9600000">
     <Initialization range="0-847"/>
     <SegmentURL mediaRange="848-9097166" indexRange="848-891"/>
    </SegmentList>
   </Representation>
  </AdaptationSet>
 </Period>
</MPD>
```

**Environment Info**
```
MP4Box -version                     
MP4Box - GPAC version 1.1.0-DEV-rev1759-geb2d1e6dd-master
(c) 2000-2022 Telecom Paris distributed under LGPL v2.1+ - http://gpac.io
	MINI build (encoders, decoders, audio and video output disabled)

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

GPAC Configuration: --static-mp4box
Features: GPAC_CONFIG_LINUX GPAC_64_BITS GPAC_HAS_SOCK_UN GPAC_MINIMAL_ODF GPAC_HAS_QJS GPAC_HAS_JPEG GPAC_HAS_PNG  GPAC_DISABLE_3D 
```

EMSG boxes are lost in fMP4 after MP4Box processing
This is a GitHub Issue
repo:gpac
owner:gpac
Title : EMSG boxes are lost in fMP4 after MP4Box processing
Issue date:

--- start body ---
Hi,

Thanks in advance for any help. Please let me know if I should send more information privately.

**Summary**
I had a `sample.mp4` file which has embedded EMSG boxes carrying custom metadata. To serve it via DASH, I used MP4Box to generate the mpd manifest file, and MP4Box generates two files for me

- `sample_dash.mpd`
- `sample_dashinit.mp4`

**Expected Result**
The EMSG boxes in the original fMP4 is preserved in `sample_dashinit.mp4` and served via `sample_dash.mpd`

**Actual Result**
The generated sample_dashinit.mp4 no longer have EMSG boxes.

**Repro Steps**

1. Take any fragmented MP4 file with EMSG boxes
2. run `MP4Box -dash 100000 sample.mp4`
3. Load the generated `sample_dashinit.mp4` using https://gpac.github.io/mp4box.js/test/filereader.html, and no EMSG boxes are in BoxView > TreeView

I might have missed any usage to preserve the EMSG boxes - can someone help advice any proper usage of MP4Box or any workaround in this case? Thanks.

Also copying the generated mpd file
```
<?xml version="1.0"?>
<!-- MPD file Generated with GPAC version 1.1.0-DEV-rev1759-geb2d1e6dd-master at 2022-02-17T06:23:14.473Z -->
<MPD xmlns="urn:mpeg:dash:schema:mpd:2011" minBufferTime="PT1.500S" type="static" mediaPresentationDuration="PT0H0M23.992S" maxSegmentDuration="PT0H0M23.993S" profiles="urn:mpeg:dash:profile:full:2011">
 <ProgramInformation moreInformationURL="http://gpac.io">
  <Title>sample_dash.mpd generated by GPAC</Title>
 </ProgramInformation>

 <Period duration="PT0H0M23.992S">
  <AdaptationSet segmentAlignment="true" maxWidth="1920" maxHeight="1080" maxFrameRate="96000/3199" par="16:9" lang="und" startWithSAP="1">
   <InbandEventStream schemeIdUri="urn:mpeg:dash:event:2012" value="1"/>
   <Representation id="1" mimeType="video/mp4" codecs="hev1.1.6.L150.0" width="1920" height="1080" frameRate="96000/3199" sar="1:1" bandwidth="3030916">
    <BaseURL>sample_dashinit.mp4</BaseURL>
    <SegmentList timescale="96000" duration="9600000">
     <Initialization range="0-847"/>
     <SegmentURL mediaRange="848-9097166" indexRange="848-891"/>
    </SegmentList>
   </Representation>
  </AdaptationSet>
 </Period>
</MPD>
```

**Environment Info**
```
MP4Box -version                     
MP4Box - GPAC version 1.1.0-DEV-rev1759-geb2d1e6dd-master
(c) 2000-2022 Telecom Paris distributed under LGPL v2.1+ - http://gpac.io
	MINI build (encoders, decoders, audio and video output disabled)

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

GPAC Configuration: --static-mp4box
Features: GPAC_CONFIG_LINUX GPAC_64_BITS GPAC_HAS_SOCK_UN GPAC_MINIMAL_ODF GPAC_HAS_QJS GPAC_HAS_JPEG GPAC_HAS_PNG  GPAC_DISABLE_3D 
```

--- end body ---
