Cache default gdk visuals in the GDK_VISUALS property on the root window

This means we don't have to try to initialize opengl in every gtk
instance that is stated. It will only happen for the first one.

https://bugzilla.gnome.org/show_bug.cgi?id=738670

gdk/x11/gdkdisplay-x11.c

@@ -165,7 +165,8 @@ static const char *const precache_atoms[] = {
   "_NET_WM_USER_TIME_WINDOW",
   "_NET_VIRTUAL_ROOTS",
   "GDK_SELECTION",
-  "_NET_WM_STATE_FOCUSED"
+  "_NET_WM_STATE_FOCUSED",
+  "GDK_VISUALS"
 };
 
 static char *gdk_sm_client_id;