Add base initialisation function

Add a semi-private function, called clutter_base_init(), which initialises
the basic Clutter functionalities (at the moment, just the GLib type system)
without calling in the backend-specific code. This function is only useful
for gtk-doc, to introspect the library when generating documentation for
signals, properties and class hierarchy.

Also, change the documentation build system to use clutter_base_init() when
launching the scanner program.

clutter/clutter-feature.c

@@ -93,7 +93,10 @@ _clutter_feature_init (void)
 gboolean
 clutter_feature_available (ClutterFeatureFlags feature)
 {
-   return (__features->flags & feature);
+  if (G_UNLIKELY (!__features))
+    return FALSE;
+
+  return (__features->flags & feature);
 }
 
 /**