Dijkstra is used to find the shortest path between two points.
In this program, use [draw/erase] to produce walls, then use [voyage] to move a ball betwen two points
using Dijkstra's algorithm. Note that vertices are created on the fly, and they just represented
jumps to any of the 8 direction (NW,N,NE,W,E,SW,S,SE) by 20 pixels. Because of that, the ball could
possibly jump through thin walls.