patch 9.2.0805: screenpos() "curscol" is wrong with 'rightleft'

patch 9.2.0805: screenpos() "curscol" is wrong with 'rightleft'

Problem:  With 'rightleft' set, the "curscol" value of screenpos() does not
          point to the screen column where the cursor is placed.
Solution: Count "curscol" from the left side of the window, leaving "col"
          and "endcol" as reading-order columns (Hirohito Higashi)

related: #20763
closes:  #20786

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Signed-off-by: Hirohito Higashi <h.east.727@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>