UI REPLACEMENT BACKGROUND BLACK SCREEN FIX V2

- Restored the proven opaque MOS V1 palette convention for custom START backgrounds.
- The selected replacement image is still exported as DRAKSTRT.MOS (or the chosen resref).
- Hide original BG2 START UI still replaces only the START menu; custom campaign buttons remain unchanged.
- Text-only/invisible button backgrounds remain unchanged.

Reason: the previous UI-specific exporter wrote a zero fourth palette byte, which BG2EE 2.7 can display as a black/transparent START mosaic.
