According to this stack overflow post: http://stackoverflow.com/questions/6777887/luabind-return-stl-iterator-for-stdmap
associative containers aren't supported by return_stl_iterator so thats a problem for lua. I tried exposing std::pair<std::pair<unsigned int, unsigned int>, Collision> to lua but didn't work. So there's two options i see: 
- Keep a seperate vector<Collision> container with the same collisions. (memory waste)
- Every time EntityFilter::collisions() is called we construct a constructor which we fill with the elements from the map and return it (terrible performance)
- ~~Use a mutable... nvm~~