Switch to sdl3

[sharkwouter]

This is still WIP. I still need to fix the following:

• Tinygettext relies on SDL2, which is annoying. I should upgrade to a version that supports SDL3.
• I should build SDL3 without X11 support on Linux. It is not that relevant anymore.
• Sound mixing is still a mess. I should split up into "channels" like before again.
• I still have to make sure all other options in the options menu work like before. There might be new bugs.
• I should run valgrind once more.
• I should fix the github action builds for all platforms. Right now they are basically all broken.
• Wii support needs to be fixed to not try to use SDL2.

There are 2 reasons for this PR:

1. SDL2 is no longer supported upstream.
2. Doing Android support should be easier with SDL3.

[sharkwouter]

@mardy do you know if the Wii has SDL3 yet?

[sharkwouter]

I have most of the builds working. I might have to drop Wii support if I don't find a way to build for it again with SDL3. I still need to fix sounds, translation support and fix some compiler warnings. I'll probably have to patch tinygettext.

[sharkwouter]

Sound mixing is still weird in a way I cannot explain. I made a bug report upstream: libsdl-org/SDL_mixer#868

[mardy]

@mardy do you know if the Wii has SDL3 yet?

No, we don't support it (yet).

[sharkwouter]

Shame, hope there will be one before the next release. Would be a shame to not have Wii support with that one.