SDL3 centring, fix Flatpak joystick, SquashFS appimage, gcc build

[mcarans]

Fixes #656
Fixes #654
Fixes #555
Fixes #659

[HiranChaudhuri]

#656 is not fixed. Hopefully the issue does not get closed when this PR gets merged for the other two issues.

[mcarans]

@oocube is the splash and game window centred for you? This PR doesn't use the separate splash launcher binary and tries to do centring with the game's built in splash.

[mcarans]

Tested on Windows and Kubuntu 26.04 - works fine.

[oocube]

@oocube is the splash and game window centred for you? This PR doesn't use the separate splash launcher binary and tries to do centring with the game's built in splash.

I tested oolite_dev-1.93.1-PullRequest658.181-x86_64.AppImage on my multi-monitor setup.
The splash screen is perfectly centered on the right (=main) screen. The main window does not come up centered.

Usually I play by moving the main screen to the center monitor and maximize it. When I stop the game and run again, the splash screen is perfectly centered on the right (=main) screen. The game window also comes up maximized but on the right (main) screen. I would have expected it to remember size and position, but my expectation may be wrong here.

[mcarans]

@oocube Thanks for testing. Where does the main window appear if it is not maximised on first launch of the game? How does that compare with behaviour on master? Did previous versions of Oolite remember position?

What is the desired behaviour here? I think it's now possible to get the particular display a window is on in a multi monitor setup so it may be possible to store it, but I'd prefer to look at that in a future PR.

For this PR if I can make things at least the similar to what 1.92.1 did regarding centring but without the separate splash launcher, then that is a start and we can look at proper multi monitor support in another PR. Is what happens in this PR and/or master worse than 1.92.1?

Here is what I found:
I tried making a dual monitor setup with my laptop and monitor using monitor as primary.

1.92.1 and current master behave largely the same with the splash and game launching centred on primary.

PullRequest658.181 launches splash and game centred on primary if I launch from a file explorer on primary or on secondary if I launch from a file explorer on secondary.

Position is not remembered on any of them, but size is.