ffmpeg: with filter_complex, avoid random in<->out mapping.

ffmpeg: with filter_complex, avoid random in<->out mapping.

With complex filters, an output can come from any input,
or several inputs, including inputs of a different type.
Copying the codec parameters from the first input with
the same type does not make any sense.

This does not change anything for simple 1->1 filters,
as source_index is set in that case.