Frederic Bouvier:

Add a static function that will be installed as a callback to
  cull or not the terrain leaves in tiles.
  Add a function to set the filter.

src/Scenery/tilemgr.cxx

@@ -55,6 +55,7 @@ queue<FGDeferredModel *> FGTileMgr::model_queue;
 #endif // ENABLE_THREADS
 queue<FGTileEntry *> FGTileMgr::delete_queue;
 
+bool FGTileMgr::tile_filter = true;
 
 // Constructor
 FGTileMgr::FGTileMgr():
@@ -512,3 +513,13 @@ void FGTileMgr::prep_ssg_nodes( SGLocation *location, float vis ) {
     }
 }
 
+bool FGTileMgr::set_tile_filter( bool f ) {
+    bool old = tile_filter;
+    tile_filter = f;
+    return old;
+}
+
+int FGTileMgr::tile_filter_cb( ssgEntity *, int )
+{
+  return tile_filter;
+}

src/Scenery/tilemgr.hxx

@@ -128,6 +128,11 @@ private:
 #endif // ENABLE_THREADS
     static queue<FGTileEntry *> delete_queue;
 
+    /**
+     * Tile filter indicator, to implement multipass rendering
+     */
+    static bool tile_filter;
+
 public:
 
     /**
@@ -188,6 +193,9 @@ public:
 
     inline SGBucket get_current_bucket () { return current_bucket; }
     inline SGBucket get_previous_bucket () { return previous_bucket; }
+
+    static bool set_tile_filter( bool f );
+    static int tile_filter_cb( ssgEntity *, int );
 };