patch 9.0.1629: having utf16idx() rounding up is inconvenient

Problem:    Having utf16idx() rounding up is inconvenient.
Solution:   Make utf16idx() round down. (Yegappan Lakshmanan, closes #12523)
diff --git a/src/version.c b/src/version.c
index 1235ac9..c753acd 100644
--- a/src/version.c
+++ b/src/version.c
@@ -696,6 +696,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    1629,
+/**/
     1628,
 /**/
     1627,