patch 8.1.1645: cannot use a popup window for a balloon

Problem:    Cannot use a popup window for a balloon.
Solution:   Add popup_beval().  Add the "mousemoved" property.  Add the
            screenpos() function.
diff --git a/src/version.c b/src/version.c
index 754155b..6165133 100644
--- a/src/version.c
+++ b/src/version.c
@@ -778,6 +778,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    1645,
+/**/
     1644,
 /**/
     1643,