Fix resize handling when enabling and disabling full-screen mode.

I'm not sure whether I broke this in the recent revamp or whether it
was always broken, but: transitions in and out of full-screen mode
work by first maximising or restoring the window, which triggers a
WM_SIZE, whose handler then fiddles with the window style to disable
or re-enable all the furniture, which in turn triggers a recursive
WM_SIZE. The trouble is, after returning from the handler for the
inner WM_SIZE, the rest of the outer handler runs, and its client area
size is now out of date.

So I've added a flag which is set when a resize is handled 'properly',
so that after returning from the inner WM_SIZE handler the outer one
knows not to try to redo badly work that's already been done well.

[originally from svn r9056]

windows/window.c

@@ -1973,6 +1973,7 @@ static LRESULT CALLBACK WndProc(HWND hwnd, UINT message,
     static int ignore_clip = FALSE;
     static int need_backend_resize = FALSE;
     static int fullscr_on_max = FALSE;
+    static int processed_resize = FALSE;
     static UINT last_mousemove = 0;
 
     switch (message) {
@@ -2756,13 +2757,36 @@ static LRESULT CALLBACK WndProc(HWND hwnd, UINT message,
 			  cfg.win_name_always ? window_name : icon_name);
 	if (wParam == SIZE_RESTORED || wParam == SIZE_MAXIMIZED)
 	    SetWindowText(hwnd, window_name);
-        if (wParam == SIZE_RESTORED)
+        if (wParam == SIZE_RESTORED) {
+            processed_resize = FALSE;
             clear_full_screen();
+            if (processed_resize) {
+                /*
+                 * Inhibit normal processing of this WM_SIZE; a
+                 * secondary one was triggered just now by
+                 * clear_full_screen which contained the correct
+                 * client area size.
+                 */
+                return 0;
+            }
+        }
         if (wParam == SIZE_MAXIMIZED && fullscr_on_max) {
             fullscr_on_max = FALSE;
+            processed_resize = FALSE;
             make_full_screen();
+            if (processed_resize) {
+                /*
+                 * Inhibit normal processing of this WM_SIZE; a
+                 * secondary one was triggered just now by
+                 * make_full_screen which contained the correct client
+                 * area size.
+                 */
+                return 0;
+            }
         }
 
+        processed_resize = TRUE;
+
 	if (cfg.resize_action == RESIZE_DISABLED) {
 	    /* A resize, well it better be a minimize. */
 	    reset_window(-1);