Fix the landcover ordering problem. Closes #15.

For detailed thinking behind the over-complicated SQL query, see the
conversation at
https://github.com/gravitystorm/openstreetmap-carto/issues/15
Remove unnecessary name column from landcover layer.
Turn off line clipping on oneway arrows.

Dasharrays and clipped lines leads to artefacts at tile boundaries.
Fix zoom levels for widest default construction lines. Closes #21