Dijkstra map should set starting depths before iterating neighbors

Thanks for the roguelike tutorial, it was a great way to get started with rust. I've been playing around with it and noticed the DijkstraMap is not quite working like I would expect.

In the DijkstraMap build function although the start tiles are [assigned 0 depth in the `open_list` queue](https://github.com/amethyst/bracket-lib/blob/master/bracket-pathfinding/src/dijkstra.rs#L153), the [depth only gets written to `dm.map` when iterating exits](https://github.com/amethyst/bracket-lib/blob/master/bracket-pathfinding/src/dijkstra.rs#L167). The effect is that a start tile's own value only gets set when iterating an adjacent tile's exits, at which point `new_depth` is 1. The start tiles then appear to have the same depth as their adjacent tiles. This is a problem for using `find_lowest_exit` to path all the way to a start tile, as the gradient disappears before actually reaching it.

For example in this image the grayscale tiles are starts and the green tiles show the distance to the gray tiles. Trying to path out of the green and into the gray tiles is unreliable because the gray exits have the same depth as some of the green exits:

Looks like the other build variations have the same issue.

I think when you push the starts into `open_list` you should also set `dm.map[*start] = 0.0;` (or the provided depth for the weighted build). Trying that gives the result I'd expect: