Investigate how to improve entity lifetime safety

[ensisoft]

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Currently the Lua based game code is handed raw pointers to entities. This works as long as the game doesn't try to do any kind of "caching" of these pointers. This can be a little bit problematic in the sense that any access to an entity / game object outside of the designated callback that handed the object could result in use after delete bug.

So far the an idea has been to use intrusive reference counting implemented in the entities. The scene and the Lua game code would be the most likely parties participating in this scheme (the engine subsystems are expected to respect lifetimes properly). The unclear question though is which mechanism to use in order to make the ref counting implicit in the Lua integration so that the Lua based game code doesn't need to do it explicitly (which is no better than the current implementation!)