patch 9.1.0446: getregionpos() inconsistent for partly-selected multibyte char

Problem:  getregionpos() behaves inconsistently for a partly-selected
          multibyte char.
Solution: Always use column of the first byte for a partly-selected
          multibyte char (zeertzjq).

closes: #14851

Signed-off-by: zeertzjq <zeertzjq@outlook.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/evalfunc.c b/src/evalfunc.c
index b389085..d53dda4 100644
--- a/src/evalfunc.c
+++ b/src/evalfunc.c
@@ -5788,6 +5788,7 @@
     for (lnum = p1.lnum; lnum <= p2.lnum; lnum++)
     {
 	pos_T		ret_p1, ret_p2;
+	char_u		*line = ml_get(lnum);
 	colnr_T		line_len = ml_get_len(lnum);
 
 	if (region_type == MLINE)
@@ -5810,7 +5811,7 @@
 	    {
 		if (region_type == MBLOCK)
 		{
-		    ret_p1.col = bd.textcol;
+		    ret_p1.col = mb_prevptr(line, bd.textstart) - line + 1;
 		    ret_p1.coladd = bd.start_char_vcols
 					     - (bd.start_vcol - oa.start_vcol);
 		}
@@ -5829,7 +5830,7 @@
 	    }
 	    else if (bd.startspaces > 0)
 	    {
-		ret_p1.col = bd.textcol;
+		ret_p1.col = mb_prevptr(line, bd.textstart) - line + 1;
 		ret_p1.coladd = bd.start_char_vcols - bd.startspaces;
 	    }
 	    else