Fix large memory leak in DRC

All elements get effective shapes, many of which get triangulated.  This
memory needs to be freed when destroying the tree

pcbnew/drc/drc_rtree.h

@@ -79,8 +79,13 @@ public:
     ~DRC_RTREE()
     {
         for( auto tree : m_tree )
+        {
+            for( auto el : *tree )
+                delete el;
+
             delete tree;
         }
+    }
 
     /**
      * Insert an item into the tree on a particular layer with an optional worst clearance.