(pos_visible_p): Be sure to move to the specified

(pos_visible_p): Be sure to move to the specified
position.  Always get the full ascent / descent of the
corresponding row, to return reliable rtop and rbot values.
(back_to_previous_visible_line_start): Fix 2005-01-18 change.
Must look one character back, as back_to_previous_line_start
returns position after the newline.
(move_it_vertically_backward): Fix heuristic for when to move further
back in case line_height * 2/3 is larger than window height.
(cursor_row_fully_visible_p): Rename make_cursor_line_fully_visible_p
as it does not do anything anymore.  Add arg current_matrix_p to
use current matrix rather than desired matrix when set.
(try_cursor_movement): Don't scroll to make cursor row fully
visible if cursor didn't move.  This avoids unexpected recentering
in case of blinking cursor or accepting process output.
Use current matrix to check cursor row visibility.
(redisplay_window): Fix whether to recenter or move to top in case
cursor line is taller than window height.
(find_first_unchanged_at_end_row): Stop search if we reach a row
which not enabled (instead of abort).