keysym attribute removed from SDL3 cause SDL3 backend build to fail

[adrian-purser]

A recent commit to SDL, "Removed SDL_Keysym", causes the SDL3 backend build to fail.

The notes in the SDL micration readme are as follows -

The keysym field of key events has been removed to remove one level of indirection, and sym has been renamed key.
Code that looked like this:

    SDL_Event event;
    SDL_Keycode key = event.key.keysym.sym;
    SDL_Keymod mod = event.key.keysym.mod;

now looks like this:

    SDL_Event event;
    SDL_Keycode key = event.key.key;
    SDL_Keymod mod = event.key.mod;

I also found that I had to change event.key.keysym.scancode to event.key.scancode

The changes in the SDL3 backend at line 346 now look like this in my fork -

case SDL_EVENT_KEY_UP:
        {
            ImGui_ImplSDL3_UpdateKeyModifiers((SDL_Keymod)event->key.mod);
            ImGuiKey key = ImGui_ImplSDL3_KeycodeToImGuiKey(event->key.key);
            io.AddKeyEvent(key, (event->type == SDL_EVENT_KEY_DOWN));
            io.SetKeyEventNativeData(key, event->key.key, event->key.scancode, event->key.scancode); // To support legacy indexing (<1.87 user code). Legacy backend uses SDLK_*** as indices to IsKeyXXX() functions.
            return true;
        }