core: Add a helper function to grab the old event mask of a window

https://bugzilla.gnome.org/show_bug.cgi?id=690581

src/core/core.c

@@ -775,3 +775,24 @@ meta_invalidate_default_icons (void)
   g_slist_free (windows);
 }
 
+void
+meta_core_add_old_event_mask (Display     *xdisplay,
+                              Window       xwindow,
+                              XIEventMask *mask)
+{
+  XIEventMask *prev;
+  gint n_masks, i, j;
+
+  prev = XIGetSelectedEvents (xdisplay, xwindow, &n_masks);
+
+  for (i = 0; i < n_masks; i++)
+    {
+      if (prev[i].deviceid != XIAllMasterDevices)
+        continue;
+
+      for (j = 0; j < MIN (mask->mask_len, prev[i].mask_len); j++)
+        mask->mask[j] |= prev[i].mask[j];
+    }
+
+  XFree (prev);
+}

src/core/core.h

@@ -205,8 +205,8 @@ void meta_core_increment_event_serial (Display *display);
 
 void meta_invalidate_default_icons (void);
 
+void meta_core_add_old_event_mask (Display     *xdisplay,
+                                   Window       xwindow,
+                                   XIEventMask *mask);
+
 #endif
-
-
-
-