Multi-instance does not work with flatpak version.

[Biguibi]

When opening multiple PPSSPP Windows, the instances cannot detect one another thus multiplayer on the same PC does not work.

Multiple instance netplay depends on shared memory as seen on this commit
hrydgard/ppsspp#14907
which fixes this issue:
hrydgard/ppsspp#14906

Enabling shared memory (device=shm) with flatseal seems to fix the issue and the instances can detect each other, which can be easily verified by having a grayed out "Backend" option and muted audio on all but the first instance.
For some reason new instances stop being detected after the "Network Initialized" message.

Off-topic note: For playing with multiple saves on Linux I would recommended creating a script to run your other instances with a different HOME variable, thus one home folder for each player, and the device=shm permission needs to be set with flatseal (or command-line) for each one of them, otherwise all instances will use the same saves which may lead to conflicts in games where all saves are within one single file.

All of this was tested with Monster Hunter Portable 3rd.