ConstA knob is a plain value in GLSurfacesConfig, at its id's path. Some knobs have no default,
and take a value of their own while unset, which the SDK's getKnob reports:
surfaces.water.season, takes season.surfaces.farmland.colors.cereal, takes the style's colour as the season
shades it.
Every surfaces knob with its kind, description, range and default. It's static data, readable without a map.