lavfi: do not call ff_filter_frame() with activate.

lavfi: do not call ff_filter_frame() with activate.

avfilter_graph_request_oldest() does work that should be done by
either the filter or the application.

The principle of this function, calling ff_request_frame() from
outside the filter was always shaky. This version is less elegant
since it requires making special cases for each filter, but it
is more robust since it no longer calls ff_request_frame()
directly without notifying the filter.

Eventually, avfilter_graph_request_oldest() will be deprecated
for a function to just run the graph.