Tweak rendering of light buildings.

* Add a border
* Make them more transparent
* Require a higher zoom to render them

The main change is adding the border, to prevent joined buildings looking like one big blob. Then I also made the rendering lighter and start at higer zoom, because those buildings are usually smaller, and by definition less important.

This should close the last item on issue #68, since the other items have been fixed by other PRs.