Upgrade SDL2 to SDL3

[Copilot]

Migrates the library from SDL2 to SDL3, addressing breaking API changes in initialization, versioning, and rendering.

Changes

Library references

• Updated DllImport from SDL2 → SDL3 and SDL2_image → SDL3_image
• Renamed Sdl2 class to Sdl
• Updated test interfaces from ISdl2* to ISdl3*

API breaking changes

• SDL_Init / SDL_InitSubSystem: Now return bool instead of int, changed from SdlResult to SdlBoolResult with UnwrapTrue()
• SDL_RenderPresent: Now returns bool, changed to SdlBoolResult with UnwrapTrue()
• Removed deprecated SDL_GetRevision() and SDL_GetRevisionNumber() functions

Before:

public static void Initialize (SdlInitializationFlags flags) 
    => UnsafeNativeMethods.SDL_Init(flags).Unwrap();  // int return

public static string Revision 
    => UnsafeNativeMethods.SDL_GetRevision().AsString();  // removed in SDL3

After:

public static void Initialize (SdlInitializationFlags flags) 
    => UnsafeNativeMethods.SDL_Init(flags).UnwrapTrue();  // bool return

// Revision property removed - no longer available in SDL3

Both main library and Image library build successfully with the changes.

Original prompt

Please upgrade to SDL 3

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