Instead of just 2 files, generalize this filter to support crossfading
arbitrarily many files. This makes the filter essentially operate similar
to the `concat` filter, chaining multiple files one after another.
Aside from just adding more input pads, this requires rewriting the
activate function. Instead of a finite state machine, we keep track of the
currently active input index; and advance it only once the current input is
fully exhausted.
This results in arguably simpler logic overall.
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