fix(ui): fix tabs not being spaced properly after virtual text with no wrap

also fixes incorrect skipping of multibyte characters

src/nvim/mbyte.c

@@ -2024,6 +2024,24 @@ int mb_charlen(const char *str)
   return count;
 }
 
+int mb_charlen2bytelen(const char *str, int charlen)
+{
+  const char *p = str;
+  int count = 0;
+
+  if (p == NULL) {
+    return 0;
+  }
+
+  for (int i = 0; *p != NUL && i < charlen; i++) {
+    int b = utfc_ptr2len(p);
+    p += b;
+    count += b;
+  }
+
+  return count;
+}
+
 /// Like mb_charlen() but for a string with specified length.
 int mb_charlen_len(const char *str, int len)
 {