Surfaceskin modification

- Made it so when rendering a model, it clones an smf to use so that data isn't overwritten
- Reimplemented the skin index property. This changes the behavior of this index if CMDL_USESURFACESKIN is activated

src/playsim/p_mobj.cpp

@@ -1368,6 +1368,7 @@ void DActorModelData::Serialize(FSerializer& arc)
 	arc("modelDef", modelDef)
 		("modelIDs", modelIDs)
 		("skinIDs", skinIDs)
+		("surfaceSkinIDs", surfaceSkinIDs)
 		("hasModel", hasModel);
 }