Put together a pretty well-performing queue using a Vector and an offset.
By using the new Vector::shift_left(int) instead of Vector::take_first()
we can avoid shifting the vector contents every time and instead only
do it every so often.
Maybe this could be generalized into a separate class, I'm not sure if it's
the best algorithm though, it's just what I came up with right now. :^)
I've used a SinglyLinkedList<Point> for the flood fill queue, since Vector
was death slow. This could definitely be made faster with a better algorithm
and/or data structure. :^)
