Subtitles m4s segments don't always work as if livesim2 fails if ttml xml fragments don't fill the entire timeline without gaps

[Murmur]

(Livesim2 from the latest master branch) Livesim2 manifest manipulation is able to modify stpp-ttml.xml fragments inside the segment files, but 500InternalError happens every few seconds. Does it fail if input ttmlx.xml file had gaps in a subtitles timing? Mp4box do writes a continuous sequence of sub_1...N.m4s segment files but not all segments carry <p> elements or multiple segments may carry the same <p> elements (render in a future or rendered in past). This is the way how mp4box does it to provide a continuous +1 numbered sequence of sub m4s files.

segmentDur=3.84s https://refapp.hbbtv.org/livesim2/tsbd_240/mup_4/spd_8/utc_keep/00_llama_h264_v9/manifest_subib_evtib_1080p.mpd segmentDur=8s https://refapp.hbbtv.org/livesim2/tsbd_240/mup_4/spd_8/utc_keep/00_llama_h264_v9_8s/manifest_subib.mpd input xml files muxed to a m4s segments, see gaps in begin-end timings https://refapp.hbbtv.org/videos/00_llama_h264_v9_8s/sub_eng.xml https://refapp.hbbtv.org/videos/00_llama_h264_v9_8s/sub_fin.xml https://refapp.hbbtv.org/videos/00_llama_h264_v9_8s/sub_swe.xml

This is what happens in LiveSim2 500 Internal Error every few segments.

Manifest
https://refapp.hbbtv.org/livesim2/tsbd_240/mup_4/spd_8/utc_keep/00_llama_h264_v9_8s/manifest_subib.mpd

https://refapp.hbbtv.org/livesim2/tsbd_99999/mup_4/spd_8/utc_keep/00_llama_h264_v9_8s/sub_fin/sub_214481481.m4s
   <p begin="476625:30:41.000" end="476625:31:03.000" ...

500:Internal Error on sub_214481482.m4s - sub_214481487.m4s files

https://refapp.hbbtv.org/livesim2/tsbd_99999/mup_4/spd_8/utc_keep/00_llama_h264_v9_8s/sub_fin/sub_214481488.m4s
   empty <tt> without subtitle divs

500:Internal Error on sub_214481489.m4s - sub_214481490.m4s files

https://refapp.hbbtv.org/livesim2/tsbd_99999/mup_4/spd_8/utc_keep/00_llama_h264_v9_8s/sub_fin/sub_214481491.m4s
  <p begin="476625:32:16.000" end="476625:32:31.000" ....