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/gui_gtk.c b/src/gui_gtk.c
index 08290a5..0b98fd4 100644
--- a/src/gui_gtk.c
+++ b/src/gui_gtk.c
@@ -1107,7 +1107,7 @@
     {
 	dragging = TRUE;
 
-	if (sb->wp != NULL)
+	if (sb->wp != NULL && GDK_IS_DRAWABLE(sb->id->window))
 	{
 	    int			x;
 	    int			y;