Merge commit 'a3a0230a'

* commit 'a3a0230a':
  avconv: init filtergraphs only after we have a frame on each input

This commit is a noop since it doesn't apply cleanly due to differences
in the dataflow between avconv and ffmpeg, and thus fixing this in the
scope of a merge is unfeasible.
Merged-by: Hendrik Leppkes <h.leppkes@gmail.com>