Latest SDL3 changes to keyboard code breaks SDL3 backend build

[adrian-purser]

The relevant SDL commit is -

The text input state has been changed to be window-specific.
SDL_StartTextInput(), SDL_StopTextInput(), SDL_TextInputActive(), SDL_ClearComposition(), and SDL_SetTextInputRect() all now take a window parameter.

This change also fixes IME candidate positioning when SDL_SetTextInputRect() is called before SDL_StartTextInput(), as is recommended in the documentation.

I don't know the correct way to get the window pointer but below is my best attempt which seems to have got me up and running in Linux.

static void ImGui_ImplSDL3_SetPlatformImeData(ImGuiViewport* viewport, ImGuiPlatformImeData* data)
{
    SDL_Window * p_window = (SDL_Window *)viewport->PlatformHandle;
    if (data->WantVisible)
    {
        SDL_Rect r;
        r.x = (int)(data->InputPos.x - viewport->Pos.x);
        r.y = (int)(data->InputPos.y - viewport->Pos.y + data->InputLineHeight);
        r.w = 1;
        r.h = (int)data->InputLineHeight;
        SDL_SetTextInputRect(p_window, &r);
        SDL_StartTextInput(p_window);
    }
    else
    {
        SDL_StopTextInput(p_window);
    }
}