Merge remote-tracking branch 'cigaes/master'

* cigaes/master:
  lavfi/af_pan: support unknown layouts on input.
  lavfi/af_pan: support unknown layouts on output.
  lswr: fix assert failure on unknown layouts.
  lavfi: parsing helper for unknown channel layouts.
  lavfi/avfiltergraph: do not reduce incompatible lists.
  lavfi/avfiltergraph: suggest a solution when format selection fails.
  lavd/lavfi: support unknown channel layouts.
  lavf/wavenc: check for a single stream.
  lavd/alsa: add stream validation
  lavd/alsa: fix timestamp calculation

Merged-by: Michael Niedermayer <michaelni@gmx.at>