patch 9.2.0614: opacity popup leaves stale cells

patch 9.2.0614: opacity popup leaves stale cells

Problem:  Background redraws under an opacity popup update ScreenLines[]
          but suppress terminal output, so the terminal no longer
          matches ScreenLines[] for those cells.  Later draws skipped
          them as "unchanged", leaving parts of the old popup on screen
          after popup_settext() or popup_clear().
Solution: Track cells whose output was suppressed under an opacity popup
          and force their next output.

fixes:  #20459
closes: #20471

Signed-off-by: Yasuhiro Matsumoto <mattn.jp@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>