HDX/AV: Animate "backtrack" in DFS

[jteresco]

It would be interesting to animate the backtrack to the next edge on top of the stack that will be considered for addition to the spanning tree in cases where we are not simply taking the one we just pushed. The color could change for one "frame" of all of the edges on the path back to the last decision point that resulted in the edge on top of the stack being pushed in the first place.