moving contours below water and other layers

[alexandervlpl]

Many thanks to all contributors for this amazing style!

This PR prevents contour lines from being drawn over water (lakes, oceans, etc).

(before)

(after)

1. This looks sloppy, like a rendering mistake.
2. Users of topo maps might think these are bathymetric contours, but they are not. ASTER/SRTM data is not intended for this. It includes very little underwater terrain and these patches are probably very inaccurate.
3. "Professional" topo maps style and label bathymetric contours differently from land ones. Example:

It would be cool and useful to include a real bathymetric layer in the future! Until then I think these land contours should not appear over water layers.

This also addresses #203 (contours over buildings).