GtkTreeViewColumn: start padding at 0

If we start with a padding of -1 then it can leak out of the size
allocation request for the column when the treeview is empty.  The
GtkTreeView will then collect these -1 values and add them together,
returning -n where 'n' is the number of columns.

This is usually not a problem because treeviews tend to be used with a
scrollbar and the width of the scrollbar will be added to this number
bringing it into positive territory again.  On Ubuntu, with overlay
scrollbars, this is not the case, however.

https://bugzilla.gnome.org/show_bug.cgi?id=703062

gtk/gtktreeviewcolumn.c

@@ -467,7 +467,7 @@ gtk_tree_view_column_init (GtkTreeViewColumn *tree_column)
   priv->button = NULL;
   priv->xalign = 0.0;
   priv->width = 0;
-  priv->padding = -1;
+  priv->padding = 0;
   priv->min_width = -1;
   priv->max_width = -1;
   priv->column_type = GTK_TREE_VIEW_COLUMN_GROW_ONLY;