patch 9.0.0913: only change in current window triggers the WinScrolled event

Problem:    Only a change in the current window triggers the WinScrolled
            event.
Solution:   Trigger WinScrolled if any window scrolled or changed size.
            (issue #11576)

src/main.c

@@ -1469,6 +1469,9 @@ main_loop(
 		time_fd = NULL;
 	    }
 #endif
+	    // After the first screen update may start triggering WinScrolled
+	    // autocmd events.  Store all the scroll positions and sizes now.
+	    may_make_initial_scroll_size_snapshot();
 	}
 #ifdef FEAT_GUI
 	if (need_mouse_correct)