Use fewer pointers: no back-pointer in tangibles.

luprex/cpp/core/world-core.cpp

@@ -147,7 +147,7 @@ World::World(WorldType wt) {
     assign_seqno_ = 1;
 }
 
-Tangible::Tangible(World *w, int64_t id) : world_(w), anim_queue_(), id_player_pool_(&w->id_global_pool_) {
+Tangible::Tangible(World *w, int64_t id) : anim_queue_(), id_player_pool_(&w->id_global_pool_) {
     plane_item_.set_id(id);
     plane_item_.track(&w->plane_map_);
 }

luprex/cpp/core/world.hpp

@@ -45,9 +45,6 @@ private:
     void deserialize(StreamBuffer *sb);
 
 public:
-    // Always points back to the world model.
-    World *world_;
-
     // Animation queue.
     //
     AnimQueue anim_queue_;