Change boundary nature reserve / national park areas

This changes the rendering of national parks in the following way:
* Render border with a double uninterrupted line (line marinas).
* Require 100px minimum size for rendering.
* For zoom levels 10-12, drop transparent fill.

* Resolves #69
* Resolves #563
* Supersedes #1077

Changes with respect to #1077:
* Restore transparent fill for zoomlevels 7-9.
* Make line slightly narrower on zoomlevels 10-12 and wider on zoomlevel 10-13.