patch 9.2.0414: Flicker when drawing window separator and pum is shown

patch 9.2.0414: Flicker when drawing window separator and pum is shown

Problem:  In a vertical split where the pum overlaps the windows vsep column,
          background draws (vsep at cursor row, status line,
          redraw_vseps, idle ins_redraw) can write into cells that are
          covered by the pum, because skip_for_popup() only protects
          those cells while pum_will_redraw is set.
Solution: In skip_for_popup(), also skip cells under a visible pum when
          the current draw is not the pum itself (screen_zindex
          POPUPMENU_ZINDEX).  Exclude the wildmenu pum (MODE_CMDLINE):
          while the cmdline grows, pum_row is briefly stale and
          protecting those cells would blank a cell of the wrapped
          cmdline row (Yasuhiro Matsumoto).

closes: #20093

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