World|Sector: Deferred calculation of sector cluster bounding boxes

It is now clear that a sector cluster can determine autonomously from
its BSP leaf data set when one or more planes need to be linked to
some other sector for "deep water hack" effects. Also, recording just
the boundary geometry of the cluster is unnecessary would not provide
any performance benefit.

doomsday/client/src/world/sector.cpp

@@ -61,15 +61,32 @@ Plane &Sector::Cluster::visPlane(int planeIndex) const
 
 AABoxd const &Sector::Cluster::aaBox() const
 {
-    // If a bounding box is assigned - use it.
-    if(!_aaBox.isNull())
+    // If the cluster is comprised of a single BSP leaf we can use the bounding
+    // box of the leaf's geometry directly.
+    if(_bspLeafs.count() == 1)
     {
-        return *_aaBox;
+        return _bspLeafs.first()->poly().aaBox();
     }
-    // Otherwise it means the cluster is comprised of a single BSP leaf, so we
-    // can use the bounding box of the leaf's geometry directly.
-    DENG_ASSERT(_bspLeafs.count() == 1); // sanity check
-    return _bspLeafs.first()->poly().aaBox();
+
+    // Time to determine bounds?
+    if(_aaBox.isNull())
+    {
+        // Unite the geometry bounding boxes of all BSP leafs in the cluster.
+        foreach(BspLeaf *leaf, _bspLeafs)
+        {
+            AABoxd const &leafAABox = leaf->poly().aaBox();
+            if(!_aaBox.isNull())
+            {
+                V2d_UniteBox((*_aaBox).arvec2, leafAABox.arvec2);
+            }
+            else
+            {
+                const_cast<Sector::Cluster *>(this)->_aaBox.reset(new AABoxd(leafAABox));
+            }
+        }
+    }
+
+    return *_aaBox;
 }
 
 Sector::Cluster::BspLeafs const &Sector::Cluster::bspLeafs() const
@@ -646,32 +663,7 @@ void Sector::buildClusters()
         }
         if(!didMerge) break;
     }
-
     // Clustering complete.
-
-    // Determine the bounds of each cluster.
-    /// @todo Defer until necessary.
-    foreach(Cluster *cluster, d->clusters)
-    {
-        // If the cluster is comprised of a single BSP leaf we can use the
-        // bounding box of it's geometry directly.
-        if(cluster->_bspLeafs.count() == 1)
-            continue;
-
-        // Unite the geometry bounding boxes of BSP leafs in the cluster.
-        foreach(BspLeaf *leaf, cluster->_bspLeafs)
-        {
-            AABoxd const &leafAABox = leaf->poly().aaBox();
-            if(!cluster->_aaBox.isNull())
-            {
-                V2d_UniteBox((*cluster->_aaBox).arvec2, leafAABox.arvec2);
-            }
-            else
-            {
-                cluster->_aaBox.reset(new AABoxd(leafAABox));
-            }
-        }
-    }
 }
 
 AABoxd const &Sector::aaBox() const