Adding backend sources for SDL3

[dmikushin]

This PR experimentally adds backend files for SDL3.

SDL3 only differs from SDL2 in some functions/enums naming and a few function arguments. Nonetheless, this PR creates separate files for SDL3 for more clearance.

I'd appreciate devs' advise what would be your preference about removing SDL2/SDL3 code deduplication. One option that I see is to use:

#if SDL_VERSION_ATLEAST(3, 0, 0)

What do you think?

[mikke89]

Very cool, thank you!

I am already feeling the maintenance overhead of all the backends, so it would be preferable if we could unite them. Especially considering that it's not just the SDL_SDLrenderer backend, but the GL ones too. It quickly becomes a combinatorial problem. We probably need some preprocessing like you suggest. Though, I think I'd prefer we use SDL_MAJOR_VERSION.

I see that they've done some work on the their high DPI support, finally! It would be really cool if we could add support for that, similar to how we do it in GLFW.

[mikke89]

Hey, do you have an update on this one? Would be great to see this being continued.

[dmikushin]

Hi @mikke89 , not yet, I've been developing some RmlUi app that shall also work as a test bed for this SDL 3.0 work. I plan to finish it, make sure no issues originate from this context, and then - clean up this PR according to your preference expressed above. Alternatively, I can refactor the PR right away, but that would be a blind shot, no real testing at all. What do you think?

[mikke89]

Cool, I'd be interested to hear more about the project of yours.

There's no hurry, but I wouldn't mind merging something not fully tested, as long as we don't have any regression for SDL2. And I'll of course help test for any such regressions. Then we can do a follow-up pull request for more polishing later on. This way other users can more easily test and help out too as they spot things.