treeview: default to width_changed = TRUE

Checking the given GtkAllocation against the current allocation insize
::size-allocate doesn't really work anymore. They are only different if
the content allocation (the one passed) and the widget allocation (the
current one) are different, so e.g. when the widget has padding >0
applied.

gtk/gtktreeview.c

@@ -2602,7 +2602,7 @@ gtk_tree_view_size_allocate (GtkWidget     *widget,
 {
   GtkTreeView *tree_view = GTK_TREE_VIEW (widget);
   GList *tmp_list;
-  gboolean width_changed = FALSE;
+  gboolean width_changed = TRUE;
   gint old_width;
   double page_size;