Special handling for hillshade and contour overlays

[hholzgra]

Most overlays want to have their data prominently on top, drawn over other features and so possibly replacing them.

Hillshade and contour layers are special in that they should be above certain layers (e.g. landuse fill patterns), but below others like e.g. POI markers; also some of the styles already come with their own such layers which should not be replaced, but potentially allowed to be turned off.

• [ ] for styles that do have their own hillshade layers toggle style status
• [ ] for styles without their own add overlay
• [ ] in the long run merge hillshade/contour into loaded style after configurable layer name

This requires some special hard coded handling in the ocitysmap code, and also some configuration extensions, es.:

(hillshade_layers|contour_layers) = ... a list of one or more bundled layer names that can be turned on or off via the status property. (hillshade_after|contour_after) ... a layer name after which hillshading or contour layers should be merged into the base style dynamically

[johsin18]

Would this fix the problem that hillshade makes the map very pale currently? This used to be different (a few years ago). See here for example: https://print.get-map.org/maps/330546/jcLQhSCAnAoKFpVf