When I try to mux a TTML XML file with base64 images, `mp4box` gives the following error:

Command:
```bash
mp4box -v -add ./en.ttml -new ./en.mp4
```

Output:
```
[TXTIn] Error parsing TTML file: Line 394 - . Abort.
[Filter] txtin (idx 3) in error / not responding properly: 3359372 consecutive errors in 1000000 us with no packet discarded or sent
	discarding all inputs and notifying end of stream on all outputs
Filters not connected:
mp4mx (importer:file=0x7f7e6b706800) (idx=1)
[Importer] Error importing ./subtitles/en/en.ttml
Failure while importing media: Feature Not Supported
Error importing ./subtitles/en/en.ttml: Feature Not Supported
```

[en.ttml.zip](https://github.com/gpac/gpac/files/15169844/en.ttml.zip)
Error while muxing TTML XML with base64 images
This is a GitHub Issue
repo:gpac
owner:gpac
Title : Error while muxing TTML XML with base64 images
Issue date:

--- start body ---
When I try to mux a TTML XML file with base64 images, `mp4box` gives the following error:

Command:
```bash
mp4box -v -add ./en.ttml -new ./en.mp4
```

Output:
```
[TXTIn] Error parsing TTML file: Line 394 - . Abort.
[Filter] txtin (idx 3) in error / not responding properly: 3359372 consecutive errors in 1000000 us with no packet discarded or sent
	discarding all inputs and notifying end of stream on all outputs
Filters not connected:
mp4mx (importer:file=0x7f7e6b706800) (idx=1)
[Importer] Error importing ./subtitles/en/en.ttml
Failure while importing media: Feature Not Supported
Error importing ./subtitles/en/en.ttml: Feature Not Supported
```

[en.ttml.zip](https://github.com/gpac/gpac/files/15169844/en.ttml.zip)
--- end body ---
