Fail to load obj file properly

How to reproduce:

1. Save obj, mtl and texture files into a directory in an external drive.
2. Start MeshLab by `flatpak run net.meshlab.MeshLab`.
3. Open obj file in step 1.
4. Got this warning message on dialog and terminal. It seems the mesh is loaded properly but texture is not loaded.

```
Warning: Error encountered while loading file: "foo.obj" Error details: Some materials definitions were not found, a default white material is used where no material was available
Opened mesh /run/user/1001/doc/4e99b84a/foo.obj in 16044 msec
All files opened in 16049 msec
```

In this example, `/mnt/hdd/mesh/foo.obj` is referenced as `/run/user/1001/doc/4e99b84a/foo.obj`. Next expected step should read `/mnt/hdd/mesh/foo.mtl` but perhaps it may try to read `/run/user/1001/doc/4e99b84a/foo.mtl`.

I checked:

- If the files are saved into usual location such as `~/Desktop`, there is no problem.
- Making a symbolic link under `~/Documents` was not helped.
- This may be caused by some sandbox restriction of flatpak. So I was trying `--filesystem=host` etc. but couldn't resolve.

OS: Ubuntu 22.04
Flatpak list:

```
Name Application ID Version Branch Installation
MeshLab net.meshlab.MeshLab 2022.02 stable system
Mesa …freedesktop.Platform.GL.default 21.3.9 21.08 system
nvidia-525-7… …op.Platform.GL.nvidia-525-78-01 1.4 system
openh264 …g.freedesktop.Platform.openh264 2.1.0 2.0 system
Yaru Gtk The… org.gtk.Gtk3theme.Yaru 3.22 system
KDE Applicat… org.kde.Platform 5.15-21.08 system
QGnomePlatfo… …de.PlatformTheme.QGnomePlatform 5.15-21.08 system
QtSNI org.kde.PlatformTheme.QtSNI 5.15-21.08 system
QGnomePlatfo… …ation.QGnomePlatform-decoration 5.15-21.08 system
```

Please tell me if this issue should be reported in MeshLab repo.

Reproduce the report with `flatpak run net.meshlab.MeshLab`, using OBJ, MTL, and texture files on an external drive. Inspect Flatpak filesystem permissions and how the application resolves the referenced MTL path; done means the MTL and texture load without the missing-material warning.

Written by the indexing model from the issue text.