More world generators support

src/voxels/WorldGenerator.h

@@ -7,6 +7,7 @@ struct voxel;
 class Content;
 
 class WorldGenerator {
+protected:
 	blockid_t const idStone;
 	blockid_t const idDirt;
 	blockid_t const idGrassBlock;
@@ -19,7 +20,7 @@ class WorldGenerator {
 	blockid_t const idBazalt;
 public:
 	WorldGenerator(const Content* content);
-	void generate(voxel* voxels, int x, int z, int seed);
+	virtual void generate(voxel* voxels, int x, int z, int seed);
 };
 
-#endif /* VOXELS_WORLDGENERATOR_H_ */
+#endif /* VOXELS_WORLDGENERATOR_H_ */