[repacker] Expose on internal method in the repacker that allows the caller to pass in/out a graph.

Will be used in testing so we can compare graphs instead of packed result.

src/graph/graph.hh

diff --git a/src/graph/graph.hh b/src/graph/graph.hh
index 5f23414..0d3dae2 100644
--- a/src/graph/graph.hh
+++ b/src/graph/graph.hh
@@ -196,7 +196,8 @@
       : parents_invalid (true),
         distance_invalid (true),
         positions_invalid (true),
-        successful (true)
+        successful (true),
+        buffers ()
   {
     num_roots_for_space_.push (1);
     bool removed_nil = false;
@@ -228,6 +229,8 @@
   ~graph_t ()
   {
     vertices_.fini ();
+    for (char* b : buffers)
+      hb_free (b);
   }
 
   bool in_error () const
@@ -255,6 +258,11 @@
     return vertices_[i].obj;
   }
 
+  void add_buffer (char* buffer)
+  {
+    buffers.push (buffer);
+  }
+
   /*
    * Adds a 16 bit link from parent_id to child_id
    */
@@ -1127,6 +1135,7 @@
   bool positions_invalid;
   bool successful;
   hb_vector_t<unsigned> num_roots_for_space_;
+  hb_vector_t<char*> buffers;
 };
 
 }