point-quadtree: Pooled quadtree provides no real performance benefits.

Increasing Quadtree.MIN_SIZE is far better for performance. By this
logic, a simple grid-partititoning scheme may be best.
point-quadtree: Add button to toggle quadtree usage.

point-quadtree: Add
zombies: Add player visibility toggle/colors. Add help text. Only prevent default key events if key is actually used.