Adding river deltas. Algorithm needs to be checked

[benskywalker]

Fix #107. Deltas seem to work

Looks good

This issue is probably unrelated.

This might be an issue.

[benskywalker]

I didn't mean to push this IsLake algorithm to this PR, but it should be fine. It kind of fits. It's mostly needed for #430, but hopefully these can both be added in fairly short order

[pcen]

This looks awesome! It would be great if you could look into rebasing onto #407, since we are in the process of updating the tile rendering to use Godot's TileMap class. The logic for choosing which tiles to render deltas on / which delta texture to use should be the same, but you'd need to add some code to load the delta pcx into a tile atlas and then index into that and add it to the tilemap

[benskywalker]

@pcen, I've been working on that. I'm closing this PR because it's easier to just make a new branch, but I resolved the issue with rivers not showing in the tilemaps branch at all. #433