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 &&

src/window.cpp

@@ -59,6 +59,8 @@ Window *_z_back_window  = NULL;
 /** If false, highlight is white, otherwise the by the widget defined colour. */
 bool _window_highlight_colour = false;
 
+uint64 _window_update_number = 1;
+
 /*
  * Window that currently has focus. - The main purpose is to generate
  * #FocusLost events, not to give next window in z-order focus when a
@@ -3096,6 +3098,8 @@ void UpdateWindows()
 {
 	Window *w;
 
+	_window_update_number++;
+
 	static int highlight_timer = 1;
 	if (--highlight_timer == 0) {
 		highlight_timer = 15;
@@ -3138,6 +3142,8 @@ void UpdateWindows()
 	NetworkDrawChatMessage();
 	/* Redraw mouse cursor in case it was hidden */
 	DrawMouseCursor();
+
+	_window_update_number++;
 }
 
 /**

src/window_gui.h

@@ -149,6 +149,18 @@ extern Window *_z_front_window;
 extern Window *_z_back_window;
 extern Window *_focused_window;
 
+inline uint64 GetWindowUpdateNumber()
+{
+	extern uint64 _window_update_number;
+	return _window_update_number;
+}
+
+inline void IncrementWindowUpdateNumber()
+{
+	extern uint64 _window_update_number;
+	_window_update_number++;
+}
+
 
 /** How do we the window to be placed? */
 enum WindowPosition {