dxgsg9: fix compile error due to WeakPointerTo changes

panda/src/dxgsg9/dxGeomMunger9.I

@@ -31,6 +31,6 @@ DXGeomMunger9(GraphicsStateGuardian *gsg, const RenderState *state) :
   }
   // Set a callback to unregister ourselves when either the Texture or the
   // TexGen object gets deleted.
-  _texture.set_callback(this);
-  _tex_gen.set_callback(this);
+  _texture.add_callback(this);
+  _tex_gen.add_callback(this);
 }

panda/src/dxgsg9/dxGeomMunger9.cxx

@@ -28,6 +28,9 @@ DXGeomMunger9::
     unref_delete(_filtered_texture);
     _reffed_filtered_texture = false;
   }
+
+  _texture.remove_callback(this);
+  _tex_gen.remove_callback(this);
 }
 
 /**