lavfi/thumbnail: support flushing.

This makes possible the raise of one picture if the input has less than
the number of frames to analyze.

Fixes Ticket1992.
lavfi/ebur128: fix format negociation for output.

Before this change, the audio input and output formats are set
independently, so the lavfi negociation could pick different settings
for the input and output. This is particularly true for the channel
layout settings, where multiple choices were available.

Fixes Ticket2342.
Revert "lavfi/ebur128: fix format negociation for output."

This reverts commit 9efcfbed9dd64679145f5f39d9a812dfeea97172.

All the shame on me; this commit is actually causing more problems
(broken outputs but also crashes) than it was solving.
lavc: do not set coded_frame->reference

That field is deprecated.

Signed-off-by: Paul B Mahol <onemda@gmail.com>