patch 9.1.0496: matched text is highlighted case-sensitively

Problem:  matched text is highlighted case-sensitively
Solution: use MB_STRNICMP, update highlighting when the base changes
          (glepnir)

fixes: #15021
closes: #15023

Co-authored-by: zeertzjq <zeertzjq@outlook.com>
Signed-off-by: glepnir <glephunter@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/popupmenu.c b/src/popupmenu.c
index fdec5f3..6e9d826 100644
--- a/src/popupmenu.c
+++ b/src/popupmenu.c
@@ -461,7 +461,7 @@
     if (in_fuzzy)
 	ga = fuzzy_match_str_with_pos(text, leader);
     else
-	matched_start = STRNCMP(text, leader, leader_len) == 0;
+	matched_start = MB_STRNICMP(text, leader, leader_len) == 0;
 
     while (*ptr != NUL)
     {