Fixed single resources being spawned at 0 density

and therefore not being rendered as described in https://github.com/OpenRA/OpenRA/issues/5565. Haos Ridges is a good test case (even without https://github.com/OpenRA/OpenRA/pull/4496) because it comes with spots showing few gems.

![image](https://cloud.githubusercontent.com/assets/756669/3208269/45a24510-ee26-11e3-9028-6a8ecf57e26f.png)