SDL 3 port

[andypiper]

Now that libSDL 3.4 is out, SDL_mixer 3 is also available. I don't think many distributions have these versions packaged yet, but now may be a good time to think about the migration steps.

With a brief bit of investigation I think the SDL_mixer stuff is the largest piece of the work.

From a Linux user perspective (my main interest) it means Wayland and modern Vulkan graphics options and Pipewire support, which is nice. Nicer windowed too. Generally a more future-proof set of APIs.

Happy to help if I can. I'm playing around with some basic migration tasks locally but I'm not yet very familiar with the OF code itself.

[segrax]

I've pushed a change switching to SDL3, its only tested on Windows so far though

[andypiper]

Great, thank you. I can do some testing on Linux and macOS soon and report back.