Add damage when restacking windows

Interesting this problem only surface when using GTK applications on
KDE. Maybe on other WMs there are some other events after window
restacking that triggers repaint of the window.

Fixes #28

Signed-off-by: Yuxuan Shui <yshuiv7@gmail.com>

src/compton.c

@@ -2142,6 +2142,9 @@ restack_win(session_t *ps, win *w, Window new_above) {
     w->next = *prev;
     *prev = w;
 
+    // add damage for this window
+    add_damage_from_win(ps, w);
+
 #ifdef DEBUG_RESTACK
     {
       const char *desc;