glsink: Enable sync meta on pools we offer

As the upload is asynchronous, we need to enable the sync meta to
gain correct rendering. The buffer pool receiver don't know about
that.

ext/gtk/gstgtkglsink.c

@@ -249,6 +249,9 @@ gst_gtk_gl_sink_propose_allocation (GstBaseSink * bsink, GstQuery * query)
 
     config = gst_buffer_pool_get_config (pool);
     gst_buffer_pool_config_set_params (config, caps, size, 0, 0);
+    gst_buffer_pool_config_add_option (config,
+        GST_BUFFER_POOL_OPTION_GL_SYNC_META);
+
     if (!gst_buffer_pool_set_config (pool, config))
       goto config_failed;