patch 9.2.0016: popup border not redrawn correctly with wide chars

patch 9.2.0016: popup border not redrawn correctly with wide chars

Problem:  When a popup window's border overwrites part of a wide
          character from another popup, the remaining half loses its
          original attribute (background color) because it is reset to 0.
Solution: Modify screen_line(), screen_puts_len(), and screen_fill() to
          preserve the existing ScreenAttrs value when clearing wide
          character boundaries. Also ensure background data is refreshed
          for transparent popup cells (Yasuhiro Matsumoto).

closes: #19299

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