player: add complex filter graph support

See --lavfi-complex option.

This is still quite rough. There's no support for dynamic configuration
of any kind. There are probably corner cases where playback might freeze
or burn 100% CPU (due to dataflow problems when interaction with
libavfilter).

Future possible plans might include:
- freely switch tracks by providing some sort of default track graph
  label
- automatically enabling audio visualization
- automatically mix audio or stack video when multiple tracks are
  selected at once (similar to how multiple sub tracks can be selected)

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