(try_window_id): Disable rows below the window end.

(try_window_id): Disable rows below the window end.
(try_window_reusing_current_matrix): Use cursor_row_p.
(try_window_reusing_current_matrix) <scrolling up>: Fix disabling
of rows.
(init_from_display_pos): If POS specifies a position in a display
vector, maybe get the iterator set up for that ellipsis.