Hello,
  I am trying to using Mp4Info to add mutiple avc1 to mp4. But something went wrong..and I dont know where is it.I would appreciate if you could help me.
 I want change the structure of mp4 
FROM :  
![20220711-144924](https://user-images.githubusercontent.com/34226411/178205394-681302c8-3cd2-40cb-bbe8-a2e9226e75c4.jpg)
 
TO:
![飞书20220711-144953](https://user-images.githubusercontent.com/34226411/178205355-5d25b4f8-3fd0-40f2-8bae-2f8844f775d4.jpg)

 Actually I just add a avc1 box to stsd and then change the sample description index in stsc box.  This is my source code
Mp4Fix.txt

  But I get a corrupt file, after checking file ,I find mdat box changed ,I donnt know why. Could you please tell me why mdat box will be changed when I modify other box? In my understanding, mdat is only used to store data. 
   I will appricate it very much if someone could give me some advices

Add multiple avc1 to MP4
This is a GitHub Issue
repo:Bento4
owner:axiomatic-systems
Title : Add multiple avc1 to MP4
Issue date:

--- start body ---
Hello,
  I am trying to using Mp4Info to add mutiple avc1 to mp4. But something went wrong..and I dont know where is it.I would appreciate if you could help me.
 I want change the structure of mp4 
FROM :  
![20220711-144924](https://user-images.githubusercontent.com/34226411/178205394-681302c8-3cd2-40cb-bbe8-a2e9226e75c4.jpg)
 
TO:
![飞书20220711-144953](https://user-images.githubusercontent.com/34226411/178205355-5d25b4f8-3fd0-40f2-8bae-2f8844f775d4.jpg)

 Actually I just add a avc1 box to stsd and then change the sample description index in stsc box.  This is my source code
Mp4Fix.txt

  But I get a corrupt file, after checking file ,I find mdat box changed ,I donnt know why. Could you please tell me why mdat box will be changed when I modify other box? In my understanding, mdat is only used to store data. 
   I will appricate it very much if someone could give me some advices

--- end body ---
