Hi,

I am encountering frequent crashes while playing back tiled VR content using the compositor, you can find the way to reproduce it below.

Encoder command:

```
 gpac\
 -i test_30fps.mp4:FID=I reframer:rt=on:SID=I:FID=REGULATED\
 ffsws:SID=REGULATED:FID=Y:osize=1024x512:ofmt=yuv420\
 -+kvazaar-params=preset=ultrafast,tiles=4x4,slices=tiles,mv-constraint=frametilemargin,input-format=P420,qp=16,rc-algorithm=lambda\
 ffenc:SID=Y:FID=K1:c=libkvazaar:b=10M:size=$SIZE:fintra=4:rc\
 tilesplit:SID=K1:FID=1 -o http://localhost:8080/live.mpd:gpac:dmode\=dynamic:segdur\=4:cdur\=2:asto\=2:llhls\=off:profile\=live:rdirs\=outdir:template\=seg_live_:SID\=1  -stats  -logs=all@info -log-file=file_to_tiles_server.log
```

Player command

``` 
gpac compositor:src=http://localhost:8080/live.mpd#VR vout:buffer=8000 -graph -logs=all@info -log-file=compositor_crash.log 
```

[Link containing](https://www.mediafire.com/file/991b9w4ilkyo9ac/compositor_crash1.zip/file):

- player log
- encoder log
- screenshot of the stacktrace
The video file used https://github.com/gpac/gpac/issues/1849

OS: ubuntu 21.04 
GPAC: version 1.1.0-DEV-rev1283-gc51f07f11-master
Compiled in debug
Ran using` _NV_PRIME_RENDER_OFFLOAD=1 __GLX_VENDOR_LIBRARY_NAME=nvidia` to make use of the nvidia GPU 

-------
Thanks for reporting your issue. Please make sure these boxes are checked before submitting your issue - thank you!

- [x] I looked for a similar issue and couldn't find any.
- [x] I tried with the latest version of GPAC. Installers available at http://gpac.io/downloads/gpac-nightly-builds/
- [x] I give enough information for contributors to reproduce my issue (meaningful title, github labels, platform and compiler, command-line ...). I can share files anonymously with this dropbox: https://www.mediafire.com/filedrop/filedrop_hosted.php?drop=eec9e058a9486fe4e99c33021481d9e1826ca9dbc242a6cfaab0fe95da5e5d95

Detailed guidelines: http://gpac.io/2013/07/16/how-to-file-a-bug-properly/

Playing with back tiled video in VR with compositor crashes
This is a GitHub Issue
repo:gpac
owner:gpac
Title : Playing with back tiled video in VR with compositor crashes
Issue date:

--- start body ---
Hi,

I am encountering frequent crashes while playing back tiled VR content using the compositor, you can find the way to reproduce it below.

Encoder command:

```
 gpac\
 -i test_30fps.mp4:FID=I reframer:rt=on:SID=I:FID=REGULATED\
 ffsws:SID=REGULATED:FID=Y:osize=1024x512:ofmt=yuv420\
 -+kvazaar-params=preset=ultrafast,tiles=4x4,slices=tiles,mv-constraint=frametilemargin,input-format=P420,qp=16,rc-algorithm=lambda\
 ffenc:SID=Y:FID=K1:c=libkvazaar:b=10M:size=$SIZE:fintra=4:rc\
 tilesplit:SID=K1:FID=1 -o http://localhost:8080/live.mpd:gpac:dmode\=dynamic:segdur\=4:cdur\=2:asto\=2:llhls\=off:profile\=live:rdirs\=outdir:template\=seg_live_:SID\=1  -stats  -logs=all@info -log-file=file_to_tiles_server.log
```

Player command

``` 
gpac compositor:src=http://localhost:8080/live.mpd#VR vout:buffer=8000 -graph -logs=all@info -log-file=compositor_crash.log 
```

[Link containing](https://www.mediafire.com/file/991b9w4ilkyo9ac/compositor_crash1.zip/file):

- player log
- encoder log
- screenshot of the stacktrace
The video file used https://github.com/gpac/gpac/issues/1849

OS: ubuntu 21.04 
GPAC: version 1.1.0-DEV-rev1283-gc51f07f11-master
Compiled in debug
Ran using` _NV_PRIME_RENDER_OFFLOAD=1 __GLX_VENDOR_LIBRARY_NAME=nvidia` to make use of the nvidia GPU 

-------
Thanks for reporting your issue. Please make sure these boxes are checked before submitting your issue - thank you!

- [x] I looked for a similar issue and couldn't find any.
- [x] I tried with the latest version of GPAC. Installers available at http://gpac.io/downloads/gpac-nightly-builds/
- [x] I give enough information for contributors to reproduce my issue (meaningful title, github labels, platform and compiler, command-line ...). I can share files anonymously with this dropbox: https://www.mediafire.com/filedrop/filedrop_hosted.php?drop=eec9e058a9486fe4e99c33021481d9e1826ca9dbc242a6cfaab0fe95da5e5d95

Detailed guidelines: http://gpac.io/2013/07/16/how-to-file-a-bug-properly/

--- end body ---
