evdev: add a way for applications to tweak how devices are opened

In some cases, applications (or actually, wayland compositors)
don't have the required permissions to access evdev directly, but
can do so with an external helper like weston-launch.

Allow them to do so with a custom callback that replaces the regular
open() path.

https://bugzilla.gnome.org/show_bug.cgi?id=704269

clutter/evdev/clutter-evdev.h

@@ -30,6 +30,23 @@
 
 G_BEGIN_DECLS
 
+/**
+ * ClutterOpenDeviceCallback:
+ * @path: the device path
+ * @flags: flags to be passed to open
+ *
+ * This callback will be called when Clutter needs to access an input
+ * device. It should return an open file descriptor for the file at @path,
+ * or -1 if opening failed.
+ */
+typedef int (*ClutterOpenDeviceCallback) (const char  *path,
+					  int          flags,
+					  gpointer     user_data,
+					  GError     **error);
+
+void  clutter_evdev_set_open_callback (ClutterOpenDeviceCallback callback,
+				       gpointer                  user_data);
+
 void  clutter_evdev_release_devices (void);
 void  clutter_evdev_reclaim_devices (void);