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.

src/mapgen_singlenode.cpp

@@ -101,8 +101,7 @@ void MapgenSinglenode::makeChunk(BlockMakeData *data)
 
 	// Calculate lighting
 	if (flags & MG_LIGHT)
-		calcLighting(node_min - v3s16(1, 0, 1) * MAP_BLOCKSIZE,
-					 node_max + v3s16(1, 0, 1) * MAP_BLOCKSIZE);
+		calcLighting(node_min, node_max);
 
 	this->generating = false;
 }