### Description

I'm translating a vtt track in an mp4 file using `gpac -i test.mp4#text -o test.ttml`. This result in an error on screen.
```Attempt to write property on input PID in filter ufvtt - ignoring
[XML] Invalid TTML doc: Invalid node stack: closing node is p but span was expected - Line 10: </p
	XML text was:
<?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:tts="http://www.w3.org/ns/ttml#styling" xmlns:ttp="http://www.w3.org/ns/ttml#parameter"  xml:lang="und">
 <head>
  <layout>
   <region xml:id="Default" tts:origin="5% 5%" tts:extent="90% 90%" tts:displayAlign="after" tts:overflow="visible" tts:textAlign="center" tts:fontSize="18px" tts:fontFamily="Serif"/>
  </layout>
 </head>
 <body>
  <div>
  <p begin="00:02:47.090" end="00:02:49.260"><span tts:fontStyle="italic">Text part 1.<br/><span tts:fontStyle="italic">- Text part 2.</span></p>
  </div>
 </body>
</tt>
```
I was able to trace it down to when there's a `<br/>` in the italic span, or when the end of the line is italic and the start of the next one as well.
TTML generation fails for italic on 2 lines
This is a GitHub Issue
repo:gpac
owner:gpac
Title : TTML generation fails for italic on 2 lines
Issue date:

--- start body ---
### Description

I'm translating a vtt track in an mp4 file using `gpac -i test.mp4#text -o test.ttml`. This result in an error on screen.
```Attempt to write property on input PID in filter ufvtt - ignoring
[XML] Invalid TTML doc: Invalid node stack: closing node is p but span was expected - Line 10: </p
	XML text was:
<?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:tts="http://www.w3.org/ns/ttml#styling" xmlns:ttp="http://www.w3.org/ns/ttml#parameter"  xml:lang="und">
 <head>
  <layout>
   <region xml:id="Default" tts:origin="5% 5%" tts:extent="90% 90%" tts:displayAlign="after" tts:overflow="visible" tts:textAlign="center" tts:fontSize="18px" tts:fontFamily="Serif"/>
  </layout>
 </head>
 <body>
  <div>
  <p begin="00:02:47.090" end="00:02:49.260"><span tts:fontStyle="italic">Text part 1.<br/><span tts:fontStyle="italic">- Text part 2.</span></p>
  </div>
 </body>
</tt>
```
I was able to trace it down to when there's a `<br/>` in the italic span, or when the end of the line is italic and the start of the next one as well.
--- end body ---
