void gtk_tree_view_get_visible_rect ( GtkTreeView* tree_view, GdkRectangle* visible_rect )
visible_rect with the currently-visible region of the
buffer, in tree coordinates. Convert to bin_window coordinates with
Tree coordinates start at 0,0 for row 0 of the tree, and cover the entire
scrollable area of the tree.
Rectangle to fill.
The argument will be set by the function. The data is owned by the instance.