patch 9.2.0492: popup: decoration wrongly drawn with clipping on border

patch 9.2.0492: popup: decoration wrongly drawn with clipping on border

Problem:  popup: clipwindow popups with border and padding could still
          spill into the surrounding chrome of the host window
Solution: Consume the border first, then the padding, per edge; spill
          any leftover clip into the opposite edge's decoration; derive
          the bottom padding row from total_height; skip the scrollbar
          branch for clipwindow popups (Yasuhiro Matsumoto).

closes: #20227

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