EMI: Use uint32 to store actor alpha mode

Storing the actor's alpha mode in a byte does not work when the value is
negative (AlphaOff == -1).
EMI: Save/restore state of costumes

EMICostume overrides the parent class's saveState/restoreState methods with
NOP functions, causing a great number of graphical glitches after restoring
a savegame. EMI costumes do not seem to have any additional dynamic state,
so the parent save and restore methods work just fine.
Merge pull request #763 from inguin/emi-fixes

EMI: Savegame fixes