patch 9.2.0572: lines disappear with wrapping virtual text after a double-width char

patch 9.2.0572: lines disappear with wrapping virtual text after a double-width char

Problem:  With 'nowrap', when a line ends with a double-width character
          exactly at the window width and has wrapping "after" virtual
          text, the lines below disappear and "@@@" is shown.
Solution: Detect that the last character fills the rightmost column using
          its displayed width (win_chartabsize(), so a <Tab> or double-width
          character is handled like a single-width one), and also when it
          overflows the last column.  Also clarify in the help that "wrap"
          only takes effect with the 'wrap' option set.

fixes:   #20384
related: #12213
closes:  #20395

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Signed-off-by: zeertzjq <zeertzjq@outlook.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>