Render tunnels on top of buildings

Tunnels under buildings are currently hard to see, because buildings are
only partially transparent. Removal of the transparency, such as in #565,
would even cause tunnels under buildings to become invisible.

Because tunnels are dashed, it is still clear that they are tunnels.

This solves #172.

This change has a couple of side effects:
- Buildings are now rendered behind highway-areas. This is an issue with
  areas with buildings in the middle. They might not all have correctly
  been tagged as multipolygon. Areas with buildings in the middle that are
  not tagged as multipolygon need to be retagged, and as long as they are
  not retagged, the houses will be hidden. Note that retagging would in
  any case be an improvement, as the building is not part of the area,
  and retagging is needed for routing (the building cannot be crossed
  for routing purposes).
- Buildings are now rendered behind citywalls, castlewalls,
  castlewalls-poly, cliffs, area-barriers, and tree-rows.
  This is probably a slight improvement.
- Buildings are now also rendered behind landuse-overlay (currently only
  military), which is not ideal, but probable acceptable. If this is a
  problem, we can push landuse-overlay further down, below tunnels.
- Buildings are now also rendered behind ferry-routes, which probably
  does not occur often.
- Buildings are now rendered behind roads-casing and
  turning-circle-casing, which is an improvement as buildings are
  already rendered behind roads-fill and turning-circle-fill.