add :block placement for single blocks

src/world/generator/WorldGenerator.hpp

@@ -79,6 +79,7 @@ class WorldGenerator {
     );
 
     void placeLine(const LinePlacement& line, int priority);
+    void placeBlock(const BlockPlacement& block, int priority);
 
     void generatePlacements(
         const ChunkPrototype& prototype, voxel* voxels, int x, int z
@@ -89,6 +90,12 @@ class WorldGenerator {
         voxel* voxels, 
         int x, int z
     );
+    void generateBlock(
+        const ChunkPrototype& prototype,
+        const BlockPlacement& placement,
+        voxel* voxels,
+        int x, int z
+    );
     void generateStructure(
         const ChunkPrototype& prototype, 
         const StructurePlacement& placement,