Fix screenshots not being saved

[mard]

This fixes the problem with screenshots (F12) not being saved anywhere.

Originally reported here: https://github.com/anegostudios/VintageStory-Issues/issues/2567

The game uses XDG_PICTURES_DIR/Vintagestory for saving screenshots, but this location was not given sandbox permissions. This change gives the game access to that directory so the screenshots can be saved.

[flathubbot]

Started test build 41670

[flathubbot]

Build 41670 successful To test this build, install it from the testing repository:

flatpak install --user https://dl.flathub.org/build-repo/24306/at.vintagestory.VintageStory.flatpakref

[jascrain]

Thank you!