Support SDL3

[Ghabry]

As the title says. Library has new name etc.

[Ghabry]

Made a quick build test and this throws hundreds of errors due to renamed API everywhere. They are really doing a huge cleanup.

There is a semantic patching script here: https://github.com/libsdl-org/SDL/blob/main/build-scripts/SDL_migration.cocci

This solves some of the errors but by far not all of them. Also does not add new required checks for return values etc.

Moving forward is likely the best to wait for first SDL3 release candidate so the API stabilizes and then create a sdl3_ui.cpp/h based on sdl2_ui.cpp/.h.

Depending on how hard it is new features can then be added to SDL3 only or backported to SDL2. Shouldn't be as hard as keeping SDL1 alive *g*

[Ghabry]

Patch to add SDL3 support: Ghabry@c41ad55

Not suitable for merge until SDL3 releases for the first time.

Relevant comment for tracking:
#1036 (comment)

[Ghabry]

Updated my sdl3 branch to build against latest SDL3 state.

Remaining issues:

• On KDE + Wayland the window is "Not responding" and stays like this but the program works (but in grayscale because it is faded out due to not responding xD)
• Sometimes audio crackling
• Segfault on exit

[Ghabry]

Updated the branch to the latest SDL3 dev version. API should be fairly stable now, they are close to releasing.

(The branch is rooted on the wii-sdl2 branch to avoid merge conflicts in CMakeLists)