(get_glyph_string_clip_rect): Fix reduction of cursor

(get_glyph_string_clip_rect): Fix reduction of cursor
height to glyph height when cursor row is not fully visible.
(make_cursor_line_fully_visible): Add FORCE_P arg to return
failure in case row is higher than window.  Callers changed.
(try_scrolling): Fix loop in scrolling if last_line_misfit (from Gerd).
Try to scroll partially visible, higher-than-window cursor row.
(redisplay_window): Always try to scroll partially visible,
higher-than-window cursor row - both initially and again with
centering_position = 0.
Clear desired matrix before retrying with centering_position = 0.