EMI: Savegame fixes

* actor.cpp uses writeByte/readByte to save the value of the enum _alpaMode which may have a negative value (AlphaOff == -1). Use a 32-bit integer instead.
* costumeemi.cpp overrides saveState/restoreState with NOPs. This is not necessary because the parent save/restore methods work just fine after adding a few missing NULL-pointer checks.