patch 9.1.0100: Redrawing can be improved with undo and 'spell'

patch 9.1.0100: Redrawing can be improved with undo and 'spell'

Problem:  When undoing with 'spell', redrawWinline() is called after
          changed_lines(), while later win_update() sets redraw type to
          UPD_NOT_VALID, even though w_redraw_top and w_redraw_bot are
          still valid.
Solution: Only set redraw type to UPD_NOT_VALID when inserting/deleting
          lines after parts of window has pending redraw, i.e., when
          changed_lines() is called after redrawWinline().
          (zeertzjq)

closes: #14019

Signed-off-by: zeertzjq <zeertzjq@outlook.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>