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 ...).

It looks like `gpac -gui` (or perhaps the underlying GLSL library) can't handle the mixed interleave format.

Works well:

```
bradh@audax:~/coding/gpac$ ./bin/gcc/gpac uncvg:c=Y8,U8,V8:interleave=comp:sampling=422:fps=0 -o ./uncompressed_yuv_comp_422.heif
bradh@audax:~/coding/gpac$ ./bin/gcc/gpac -gui ./uncompressed_yuv_comp_422.heif 
[Thread gf_aout] Couldn't set priority(2) for thread ID 0xc67b7640
```

Fails:

```
bradh@audax:~/coding/gpac$ ./bin/gcc/gpac uncvg:c=Y8,U8,V8:interleave=mix:sampling=422:fps=0 -o ./uncompressed_yuv_mix_422.heif
bradh@audax:~/coding/gpac$ ./bin/gcc/gpac -gui ./uncompressed_yuv_mix_422.heif 
[Thread gf_aout] Couldn't set priority(2) for thread ID 0x06045640
[V3D:GLSL] Unknown pixel format YUV4
```

This is on ubuntu. libglx-dev is  1.4.0-1.
gpac gui incompatible with uncompressed mixed subsampling
This is a GitHub Issue
repo:gpac
owner:gpac
Title : gpac gui incompatible with uncompressed mixed subsampling
Issue date:

--- start body ---
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 ...).

It looks like `gpac -gui` (or perhaps the underlying GLSL library) can't handle the mixed interleave format.

Works well:

```
bradh@audax:~/coding/gpac$ ./bin/gcc/gpac uncvg:c=Y8,U8,V8:interleave=comp:sampling=422:fps=0 -o ./uncompressed_yuv_comp_422.heif
bradh@audax:~/coding/gpac$ ./bin/gcc/gpac -gui ./uncompressed_yuv_comp_422.heif 
[Thread gf_aout] Couldn't set priority(2) for thread ID 0xc67b7640
```

Fails:

```
bradh@audax:~/coding/gpac$ ./bin/gcc/gpac uncvg:c=Y8,U8,V8:interleave=mix:sampling=422:fps=0 -o ./uncompressed_yuv_mix_422.heif
bradh@audax:~/coding/gpac$ ./bin/gcc/gpac -gui ./uncompressed_yuv_mix_422.heif 
[Thread gf_aout] Couldn't set priority(2) for thread ID 0x06045640
[V3D:GLSL] Unknown pixel format YUV4
```

This is on ubuntu. libglx-dev is  1.4.0-1.
--- end body ---
