patch 8.2.4713: plugins cannot track text scrolling

Problem:    Plugins cannot track text scrolling.
Solution:   Add the WinScrolled event. (closes #10102)
diff --git a/src/structs.h b/src/structs.h
index b8648a5..738c592 100644
--- a/src/structs.h
+++ b/src/structs.h
@@ -3510,6 +3510,12 @@
 				    // window
 #endif
 
+    // four fields that are only used when there is a WinScrolled autocommand
+    linenr_T	w_last_topline;	    // last known value for w_topline
+    colnr_T	w_last_leftcol;	    // last known value for w_leftcol
+    int		w_last_width;	    // last known value for w_width
+    int		w_last_height;	    // last known value for w_height
+
     /*
      * Layout of the window in the screen.
      * May need to add "msg_scrolled" to "w_winrow" in rare situations.