window: Fix delayed mouse mode on X

On X, basing the check whether the pointer is on the window on
Clutter events does not work, as the relevant events are handled
by GDK instead.
So add an X-specific window_has_pointer() implementation to also
fix mouse mode when running as X compositor.

https://bugzilla.gnome.org/show_bug.cgi?id=730541

src/core/window.c

@@ -7699,7 +7699,7 @@ mouse_mode_focus (MetaWindow  *window,
 }
 
 static gboolean
-window_has_pointer (MetaWindow *window)
+window_has_pointer_wayland (MetaWindow *window)
 {
   ClutterDeviceManager *dm;
   ClutterInputDevice *dev;
@@ -7713,6 +7713,39 @@ window_has_pointer (MetaWindow *window)
   return pointer_actor && clutter_actor_contains (window_actor, pointer_actor);
 }
 
+static gboolean
+window_has_pointer_x11 (MetaWindow *window)
+{
+  MetaDisplay *display = window->display;
+  MetaScreen *screen = window->screen;
+  Window root, child;
+  double root_x, root_y, x, y;
+  XIButtonState buttons;
+  XIModifierState mods;
+  XIGroupState group;
+
+  meta_error_trap_push (display);
+  XIQueryPointer (display->xdisplay,
+                  META_VIRTUAL_CORE_POINTER_ID,
+                  screen->xroot,
+                  &root, &child,
+                  &root_x, &root_y, &x, &y,
+                  &buttons, &mods, &group);
+  meta_error_trap_pop (display);
+  free (buttons.mask);
+
+  return meta_display_lookup_x_window (display, child) == window;
+}
+
+static gboolean
+window_has_pointer (MetaWindow *window)
+{
+  if (meta_is_wayland_compositor ())
+    return window_has_pointer_wayland (window);
+  else
+    return window_has_pointer_x11 (window);
+}
+
 static gboolean
 window_focus_on_pointer_rest_callback (gpointer data)
 {