SDL3 backend

[ghost]

There are already projects leveraging SDL3, e.g. https://github.com/rhett-lee/nim_duilib

[SpartanJ]

Yes, it's still not stable enough for my taste, I'm locally using ecode with SDL3 (using the compat layer) and I've found several issues during its use, I reported some of them, I'll continue monitoring until I feel comfortable enough to port it. For the moment there's no reason to do it, all features needed are preset in both libraries.