Add a monotonically incrementing counter to UpdateWindows calls

This is for checking draw cache validity

src/viewport.cpp

@@ -369,6 +369,8 @@ static Point _vp_move_offs;
 
 static void DoSetViewportPosition(const Window *w, int left, int top, int width, int height)
 {
+	IncrementWindowUpdateNumber();
+
 	FOR_ALL_WINDOWS_FROM_BACK_FROM(w, w) {
 		if (left + width > w->left &&
 				w->left + w->width > left &&