patch 9.1.0423: getregionpos() wrong with blockwise mode and multibyte

Problem:  getregionpos() wrong with blockwise mode and multibyte.
Solution: Use textcol and textlen instead of start_vcol and end_vcol.
          Handle coladd properly (zeertzjq).

Also remove unnecessary buflist_findnr() in add_regionpos_range(), as
getregionpos() has already switched buffer.

closes: #14805

Signed-off-by: zeertzjq <zeertzjq@outlook.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

src/version.c

@@ -704,6 +704,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    423,
 /**/
     422,
 /**/