(init_iterator): Clear it.max_extra_line_spacing.

(init_iterator): Clear it.max_extra_line_spacing.
(move_it_vertically_backward): Don't recure to move further back.
(move_it_vertically): Remove superfluous condition.
(move_it_by_lines): Clear last_height when moved 0 lines.
(resize_mini_window): use it.max_extra_line_spacing.
(display_tool_bar_line): Clear row->extra_line_spacing.
(try_scrolling): Use move_it_vertically_backward directly.
(redisplay_window): Likewise.
(compute_line_metrics): Set row->extra_line_spacing.
(display_line, display_string): Likewise.
(x_produce_glyphs): Update it->max_extra_line_spacing.