f_decoder_wrapper: add support for N -> 1 stream merging

This allows to spin N decoder of dependent streams, and merge them in
single output using a filter. Currently only lavfi is supported, and
should be set as a string in the stream group definition.

In the future more advance merging can be implemented in place of lavfi,
but the grouping of the streams should be similar.

This will be useful for tile merging or LCEVC style enchantment support.
May even be useful for DV EL, but this is often not a separate track.