Replace sdl2 to sdl2-compat.

[AiraYumi]

Description

Since sdl3 has been released, it is expected that sdl2 will not be actively released for updates compared to sdl3.
Therefore, we propose to replace sdl2 with sdl2-compat.
Although sdl2-compat is named sdl2, sdl3 is used internally.
With sdl2-compat, you can use the features of sdl3 without much modification to the source code that uses sdl2.

[AiraYumi]

This should be easier to change than migrating from sdl2 to sdl3 straight away.
https://github.com/libsdl-org/sdl2-compat/

[AiraYumi]

Testing Code

https://github.com/AiraYumi/3p-sdl2/releases/tag/v2.32.50-r8
https://github.com/AiraYumi/3p-open-libndofdev/releases/tag/v1.14-r200

[AiraYumi]

Testing Code

https://github.com/AiraYumi/viewer/tree/use_sdl2-compat
https://github.com/AiraYumi/viewer/actions/runs/13473917526

[AiraYumi]

Once the Linux-related code is restored to the "release/2025.03" branch, I plan to move to development on that branch.
Would it be better to do this when it is merged into the develop branch?

[AiraYumi]

Closing due to plans to migrate to sdl3.