Add scene filter which provides the same functionality as the vout_output image, but which can be inserted in a playback or stream out chain.

This will help our muxers creating better files.

This function was returning a buffer allocated with the wrong type (alloca)
when a filter returned 0 sample (scaletempo at least).

unmaintained and confuse users.

I forgot that changes in [f351451d]

(access_filter_dump could probably be removed too)

The record module:

- automatically selects the right muxer and filename extension by
looking the ES codec. In case it fails, it probes directly the
muxers and keeps the better one.

- removes all ES packets until the first key frame.

- truncates the ES without key frame to match the start time of
the other tracks.

The record access_filter will become useless as soon as all demuxers that can
support it will be modified.
The record support is half done. I will also add es_out_t record functionnalities,
and a way to selected one of them if both are supported.