lavfi: replace avfilter_open() with avfilter_graph_alloc_filter().

lavfi: replace avfilter_open() with avfilter_graph_alloc_filter().

Since we do not support "standalone" filters not attached to an
AVFilterGraph, we should not have a public function to create such
filters. In addition that function is horribly named, the action it does
cannot be possibly described as "opening" a filter.