stage-view: Fix memory handling of :framebuffer property

Currently the setter doesn't take ownership of the value, but dispose()
will unref it (and thus release someone else's reference). Fix this by
taking ownership of the property value in the setter.

clutter/clutter/clutter-stage-view.c

@@ -139,7 +139,7 @@ clutter_stage_view_set_property (GObject      *object,
       priv->layout = *layout;
       break;
     case PROP_FRAMEBUFFER:
-      priv->framebuffer = g_value_get_boxed (value);
+      priv->framebuffer = g_value_dup_boxed (value);
       break;
     }
 }