The cause of a single light source seemingly being lit without spread
was due to its creation in the +Y mapblock boundary layer during map
generation, which was ignored as the overtop. This overtop explicitly
needs to be omitted during sunlight propagation, however. To accomplish
this, Mapgen::calcLighting() was split into separate functions taking
separate parameters.
Additionally, do not diminish light too early during spread. This fixes the
output inconsistency between Map::updateLighting and Mapgen::calcLighting.
MapgenParams is no longer a polymorphic class, eliminating the need for messy and bug-prone reallocations.
Separation between the common and mapgen-specific parameters is now strongly defined.
Mapgen parameters objects are now properly encapsulated within the proper subsystems.