lavfi: add test for concat.
lavfi/af_amerge: return EAGAIN if the input layouts are not known.

Also downgrade the error to a warning.
lavfi: support multiple rounds of format negotiation.

Remove the temporary hack for amerge and replace it with a
generic solution.
lavf/mux: add the flush_packets option.

Note: a lot of muxers already do the flushing explicitly.