Lua sometimes crashing on application exit

Now that Lua respects C++ reference counts, the shutdown order is switched (Lua shutdown before context is released and Core shutdown). This PR fixes that for the Lua Invaders sample.

Sometimes (like in the case of the luainvaders sample), some of the C++ references to Lua (ex LuaEventListener) would stick around after the Lua interpreter had shutdown, which crashed the application when exited. This should fix that issue as well.