Added A_SpriteOffset. (#895)

- Coordinates work akin to A_OverlayOffset: +X shifts to the right, +Y shifts down.

src/playsim/p_mobj.cpp

@@ -365,6 +365,7 @@ void AActor::Serialize(FSerializer &arc)
 		A("spawntime", SpawnTime)
 		A("spawnorder", SpawnOrder)
 		A("friction", Friction)
+		A("SpriteOffset", SpriteOffset)
 		A("userlights", UserLights);
 
 		SerializeTerrain(arc, "floorterrain", floorterrain, &def->floorterrain);