Allow setting the default backend from the configuration file

We should allow a configuration file to set up the initial state of the
global state, which also implies being able to set the backend.

If the allowed backends have already been set programmatically via the
clutter_set_windowing_backend(), though, then the application code takes
precedence, as we assume that the application author knows better than
us what their code supports or requires.

clutter/clutter-backend-private.h

@@ -155,6 +155,8 @@ void                    _clutter_backend_reset_cogl_framebuffer         (Clutter
 
 void                    clutter_set_allowed_drivers                     (const char             *drivers);
 
+void                    clutter_try_set_windowing_backend               (const char             *drivers);
+
 G_END_DECLS
 
 #endif /* __CLUTTER_BACKEND_PRIVATE_H__ */

clutter/clutter-backend.c

@@ -1527,6 +1527,13 @@ clutter_set_windowing_backend (const char *backend_type)
   allowed_backends = g_strdup (backend_type);
 }
 
+void
+clutter_try_set_windowing_backend (const char *backend_type)
+{
+  if (allowed_backends == NULL)
+    clutter_set_windowing_backend (backend_type);
+}
+
 PangoDirection
 _clutter_backend_get_keymap_direction (ClutterBackend *backend)
 {

clutter/clutter-main.c

@@ -221,6 +221,17 @@ clutter_config_read_from_key_file (GKeyFile *keyfile)
   if (!g_key_file_has_group (keyfile, ENVIRONMENT_GROUP))
     return;
 
+  str_value =
+    g_key_file_get_string (keyfile, ENVIRONMENT_GROUP,
+                           "Backends",
+                           &key_error);
+  if (key_error != NULL)
+    g_clear_error (&key_error);
+  else
+    clutter_try_set_windowing_backend (str_value);
+
+  g_free (str_value);
+
   str_value =
     g_key_file_get_string (keyfile, ENVIRONMENT_GROUP,
                            "Drivers",