UI REPLACEMENT BACKGROUND BLACK-SCREEN FIX

When "Hide original BG2 START UI" is enabled, the generated START menu now:
- keeps the selected custom MOS as the actual START background;
- does not call Infinity_SetBackground('BACKGROUND'), which could leave the replacement START screen black after stock artwork was removed;
- uses the same direct MOS rendering form as the stock BG2EE START screen (no scaleToClip on START).

Campaign/button logic is unchanged.
Text-only/invisible button backgrounds are unchanged.
