We can embed the information we need into the silhouette map
by adding two boolean flags in each cell. One boolean will tell us
whether the western edge has a boundary and the other will
do the same for the northern boundary.
In this example, the west edge is a "true" because we have an edge there and the north edge is a "false" because there is no edge there.
We can determine the status of the other two edges by looking at the neighboring cells...