add 'lowering' generated structures property

src/content/loading/GeneratorLoader.cpp

@@ -127,7 +127,7 @@ static VoxelStructureMeta load_structure_meta(
 ) {
     VoxelStructureMeta meta;
     meta.name = name;
-
+    config.at("lowering").get(meta.lowering);
     return meta;
 }
 

src/world/generator/GeneratorDef.hpp

@@ -15,7 +15,10 @@ class VoxelFragment;
 struct GeneratorDef;
 
 struct VoxelStructureMeta {
+    /// @brief Structure name
     std::string name;
+    /// @brief Structure lowering on placement
+    int lowering = 0;
 };
 
 struct BlocksLayer {

src/world/generator/WorldGenerator.cpp

@@ -270,10 +270,11 @@ void WorldGenerator::generateStructures(
             if (height < def.seaLevel) {
                 continue;
             }
-            auto& structure = *def.structures[structureId]->fragments[rotation];
-            glm::ivec3 position {x, height, z};
-            position.x -= structure.getSize().x / 2;
-            position.z -= structure.getSize().z / 2;
+            auto& structure = *def.structures[structureId];
+            auto& fragment = *structure.fragments[rotation];
+            glm::ivec3 position {x, height-structure.meta.lowering, z};
+            position.x -= fragment.getSize().x / 2;
+            position.z -= fragment.getSize().z / 2;
             placeStructure(
                 StructurePlacement {
                     structureId,