patch 8.2.4599: GTK: get assertion errors when scrolling a split window

Problem:    GTK: get assertion errors when scrolling a split window.
Solution:   Use GDK_IS_DRAWABLE() on the scrollbar window. (closes #9982)
diff --git a/src/version.c b/src/version.c
index 1e02a58..0a6c036 100644
--- a/src/version.c
+++ b/src/version.c
@@ -751,6 +751,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    4599,
+/**/
     4598,
 /**/
     4597,