wayland: Don't "probe" for a Wayland compositor if started from compositor

In the case that the client is started directly by the compositor the
WAYLAND_SOCKET environment variable is set containing the fd to use that was
created by a socketpair.

This environment variable is consumed by a call to wl_display_connect so a
second call will not take advantage of it.

https://bugzilla.gnome.org/show_bug.cgi?id=697673

gdk/wayland/gdkdisplaymanager-wayland.c

@@ -59,6 +59,12 @@ gdk_wayland_display_manager_initable_init (GInitable     *initable,
 {
   struct wl_display *wl_display;
 
+  /* Set by the compositor when launching a special client - and it gets reset
+   * by wl_display_connect so we must avoid calling it twice
+   */
+  if (g_getenv ("WAYLAND_SOCKET"))
+    return TRUE;
+
   /* check that a connection to the default display is possible */
   wl_display = wl_display_connect (gdk_get_display_arg_name ());