Linux splash borderless

[mcarans]

@LW-archlinux this makes the splash screen borderless and ensures that when you exit fullscreen (F12) the window is centred on Linux.
@phkb This was what you tested before

Packages here: https://github.com/OoliteProject/oolite/actions/runs/31907530911?pr=714

Fixes #712

[LW-archlinux]

Patch doesn't apply cleanly to current master caea422 , probably because #711 overlaps with the commits here.

Try rebasing this and just add the 4th commit to master ?

[mcarans]

Patch doesn't apply cleanly to current master caea422 , probably because #711 overlaps with the commits here.

Try rebasing this and just add the 4th commit to master ?

@LW-archlinux Ok I've done that

(packages here: https://github.com/OoliteProject/oolite/actions/runs/31910583465?pr=714)

[LW-archlinux]

No bad side effects, switching between window / full screen works flawless. Size of window is remembered, position is centered after switch as intended.