[SEVERE] Fixed incredibly rare crash bug where ~SOMETIMES~ (yay, C++), when creating new entities duing an update loop, the application would unexplicably crash, giving out google::sparsehash errors. I now create a copy of the entities to update, so that the the looped entity vector won't get modified. 
Considerations: 
* What happens when an entity's update method causes an entity to die, and then it gets updated later?
* ^ Should an "alive" bool flag be added to entities to prevent updating dead entities?