Drop polygon-opacity from landcover

Drop polygon-opacity from landcover tags that still had it, i.e.:
* tourism = camp_site
* tourism = caravan_site
* tourism = picnic_site
* landuse = garages
* landuse = fill
* leisure = park
* leisure = recreation_ground
* landuse = brownfield
* landuse = landfill
* landuse = construction
* aeroway = aerodome

Polygon-opacity should be avoided, as it gives smudgy and sometimes unexpected
colours in case of overlapping polygons.

Also colour landuse=field as landuse=farm_land, as the old landuse=field
without opacity is too dark.

Resolves #461.