Have Fl_Pack::draw() call Fl_Group::init_sizes() on its parent group.

Fl_Pack::draw() sometimes resizes itself. This must be followed
by a call to Fl_Group::init_sizes() as indicated in that function's doc:
"If you rearrange the widgets in your group, call this method to register
the new arrangement with the Fl_Group that contains them."

src/Fl_Pack.cxx

@@ -135,6 +135,8 @@ void Fl_Pack::draw() {
   th += Fl::box_dh(box()); if (th <= 0) th = 1;
   if (tw != w() || th != h()) {
     Fl_Widget::resize(x(),y(),tw,th);
+    Fl_Group *parent = this->parent();
+    if (parent) parent->init_sizes();
     d = FL_DAMAGE_ALL;
   }
   if (d&FL_DAMAGE_ALL) {