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.

Spotted-by: Cristian Secară

Preliminary changes to fix postprocessing. Decoder quantizer settings can now be exported in the picture_t struct. Currently only works if the video is being decoded by ffmpeg since other codecs don't expose quantizer stuff. Please review.

Seems like it is too slow on Win32 (??). Hopefully fixes #1919.