patch 9.2.0896: textprop: wrong Tab size in a line with virtual text above it

patch 9.2.0896: textprop: wrong Tab size in a line with virtual text above it

Problem:  In a line with virtual text above it a Tab does not have the right
          size, depending on the width of the window.
Solution: Do not count the columns of the virtual text for the size of the
          Tab, neither when drawing nor when computing the column.

fixes:  #12232
closes: #20901

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Signed-off-by: Hirohito Higashi <h.east.727@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>