Work around access to global resources without calling gtk_init()

Some utilities such as GIR and gtk-doc, initialize class vtables without
initializing GTK+, with composite templates accessing resources this
causes a hand full of unneeded warnings.

The workaround for now is the use a private function _gtk_ensure_resources()
which is both called while initializing GTK+, and at the beginning of
gtk_widget_class_set_template_from_resource() (the private function
ensures that the resource will only ever be registered GOnce).

gtk/gtkprivate.c

@@ -30,6 +30,7 @@
 #include "gdk/gdk.h"
 
 #include "gtkprivate.h"
+#include "gtkresources.h"
 
 
 #if !defined G_OS_WIN32 && !(defined GDK_WINDOWING_QUARTZ && defined QUARTZ_RELOCATION)
@@ -251,3 +252,18 @@ _gtk_translate_keyboard_accel_state (GdkKeymap       *keymap,
 
   return retval;
 }
+
+static gpointer
+register_resources (gpointer data)
+{
+  _gtk_register_resource ();
+  return NULL;
+}
+
+void
+_gtk_ensure_resources (void)
+{
+  static GOnce register_resources_once = G_ONCE_INIT;
+
+  g_once (&register_resources_once, register_resources, NULL);
+}