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.
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