Bug in ECS::removeEntity

[ScottAlexanderCameron]

Line 49 of ecs.cpp moves the last entity pointer of the array to the index where the entity being removed currently is. The first element of the pair representing the entity is supposed to be the index in the array where that entity lives. The entity moved on this line now lives at index destIndex, while the first element of the pair is still srcIndex. Perhaps after the move, there should also be the following assignment:

entities[destIndex]->first = destIndex;