Lighting: Fix nearly all issues

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.