Mgvalleys: Fix missing decorations and incorrect function order

Fix missing decorations at horizontal chunk borders by adding
'updateHeightmap()' after terrain generation.

Swap order of 'calculateNoise' and 'calcBiomeNoise' because
'calculateNoise' modifies the heat and humidity maps created in
'calcBiomeNoise'.

Remove confusing comment, code block is not just for mods and
seems essential for correct mapgen behaviour.

src/mapgen_valleys.cpp

@@ -238,17 +238,21 @@ void MapgenValleys::makeChunk(BlockMakeData *data)
 
 	blockseed = getBlockSeed2(full_node_min, seed);
 
-	// Generate noise maps and base terrain height.
-	calculateNoise();
-
 	// Generate biome noises.  Note this must be executed strictly before
 	// generateTerrain, because generateTerrain depends on intermediate
 	// biome-related noises.
 	m_bgen->calcBiomeNoise(node_min);
 
+	// Generate noise maps and base terrain height.
+	// Modify heat and humidity maps.
+	calculateNoise();
+
 	// Generate base terrain with initial heightmaps
 	s16 stone_surface_max_y = generateTerrain();
 
+	// Recalculate heightmap
+	updateHeightmap(node_min, node_max);
+
 	// Place biome-specific nodes and build biomemap
 	MgStoneType stone_type = generateBiomes();
 
@@ -549,10 +553,6 @@ int MapgenValleys::generateTerrain()
 			index_3d += ystride;
 		}
 
-		// This happens if we're generating a chunk that doesn't
-		// contain the terrain surface, in which case, we need
-		// to set heightmap to a value outside of the chunk,
-		// to avoid confusing lua mods that use heightmap.
 		if (heightmap[index_2d] == -MAX_MAP_GENERATION_LIMIT) {
 			s16 surface_y_int = myround(surface_y);
 			if (surface_y_int > node_max.Y + 1 || surface_y_int < node_min.Y - 1) {