feat: add dual SDL2/SDL3 support with CMake option

[doncollins1985]

• Add USE_SDL CMake option (Auto/SDL2/SDL3) — prefers SDL3 when available
• All source files use #ifdef USE_SDL3 guards for API differences: Event types, key struct members, modifier keys, timer/window functions, ImGui backend (sdl2/sdl3), GLSL version (150/300 es), audio capture API, OpenGL proc loader, drop file handling
• Conditional SDL linkage: SDL3::SDL3 vs SDL2::SDL2 + SDL2::SDL2main
• New cmake/SDL3Target.cmake for SDL3 target verification
• Updated ImGui.cmake for conditional backend and kept ImGuiDemo target
• Updated dependencies_check.cmake and packaging-linux.cmake for SDL version
• Added audio device hotplug support for SDL3
• Added CurrentAudioLevel() and RefreshDeviceList() to AudioCapture
• Refactored drop file handler into HandleDropFile() method
• SDL3 audio capture uses SDL_AudioStream API
• SDL key constants remapped via #undef/#define for SDL3 naming
• Guarded projectm_create_with_opengl_load_proc for older libprojectM
• CI builds both SDL2 and SDL3 on all four platforms: Ubuntu Linux, Arch Linux, Windows (vcpkg), macOS (Homebrew)
• vcpkg.json includes both sdl2 and sdl3
• Windows runner pinned to windows-2022 for VS 2022 compatibility

[doncollins1985]

?

[kblaschke]

Overall, a solid upgrade.

I'm not a huge fan of using #ifdefs everywhere, though given how SDL calls are spread over the code base, I don't see any other simple solution than wrapping SDL again, which doesn't make lots of sense.

I've added some comments, mostly related to the build files, packaging and check workflow. The most important one is adding features to the vcpkg manifest to prevent pulling in both libraries when only one of them is ever needed.

[kblaschke]

Sorry for the late reply, I was swamped with other work in the past weeks/months, and the PRs just kept piling up.
You've not been forgotten!

[kblaschke]

Looks good except for the unused CurrentAudioLevel() functions, which should be removed as they seemingly originate from another feature not part of this PR.

The other comments are just minor things, I leave it to you to change them or leave it as-is.

[kblaschke]

You can call the AudioInputCallback() function here to avoid duplicate code:

Suggested change

	unsigned int samples = bytesRead / sizeof(float) / _channels;
	projectm_pcm_add_float(_projectMHandle, buffer.data(), samples,
	static_cast<projectm_channels>(_channels));
	AudioInputCallback(this, buffer.data(), bytesRead);

Or use the same synchronous technique with both SDL2 and SDL3 via SDL_AudioStreamAvailable()/SDL_AudioStreamGet() for SDL2 here.

[kblaschke]

Just as a thought:

Instead of all these redefines, it might be easier to declare our own enum with the key codes in two separate headers, one with SDL2 and one with SDL3 constants, and then include the correct one at the top, then use the enum values instead of the SDL2 constants.

Later, we'll support custom key bindings, so mapping all key constants would be necessary anyway.

We can keep this for now, I'll refactor it when I start the key binding stuff.

[kblaschke]

Suggested change

	#ifdef USE_SDL3
	return "SDL3 Rendering Window";
	#else
	return "SDL2 Rendering Window";
	#endif
	return "SDL Rendering Window";

That's probably sufficient. Devs can still easily check which SDL library the executable is linked against if needed.

[kblaschke]

I don't see this function being used anywhere, and it's not doing anything useful.
Please remove this function and the other copies in the *Impl classes.