Fix SDL2 incomplete closedown causing crash on quit for AmigaOS 4

[Copilot]

Fixes issue #744 where calling just SDL_Quit() is insufficient for proper SDL cleanup on AmigaOS 4, leading to a recoverable alert about "not all resources freed" that hangs both the Stratagus application and the shell/Workbench task from which it was started.

Problem

The current SDL cleanup implementation only calls SDL_Quit() without properly destroying SDL objects first:

// Current problematic cleanup
atexit(SDL_Quit);  // In normal exit
SDL_Quit();        // In debug signal handler

On AmigaOS 4, the OpenGL implementation requires all SDL resources to be explicitly freed before calling SDL_Quit(), otherwise it triggers a system alert that blocks the application.

Solution

Implemented a comprehensive CleanUpVideoSdl() function that properly destroys all SDL resources in the correct order before calling SDL_Quit():

static void CleanUpVideoSdl()
{
    if (TheRenderer) {
        SDL_DestroyRenderer(TheRenderer);
        TheRenderer = nullptr;
    }

    if (TheWindow) {
        SDL_DestroyWindow(TheWindow);
        TheWindow = nullptr;
    }

    if (Video.blankCursor) {
        Video.blankCursor.reset();
    }

    SDL_StopTextInput();
    SDL_Quit();
}

The function is now used in both normal exit (atexit(CleanUpVideoSdl)) and debug signal handlers, ensuring complete resource cleanup in all scenarios.

Changes

• Added: CleanUpVideoSdl() static function with proper resource cleanup sequence
• Updated: Normal exit handler to use new cleanup function instead of raw SDL_Quit()
• Updated: Debug signal handler to use new cleanup function for consistent behavior

This implementation follows the exact solution suggested in the issue and should resolve the hanging problem on AmigaOS 4 while maintaining compatibility with all other platforms.

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[timfel]

@Jarod42 I hope you don't mind, I got curious and wanted to try this github thingy ;-)

[Jarod42]

No issues to test it (and even merge it if appropriate).