lavfi: loop on request_frame if necessary.

Some filters need several input frames before producing output.
For these filter, it becomes simpler to return 0 in
request_frame() and let the framework call it again until
output has been produced.
lavfi: reindent after last commit.
lavfi/vf_tile: simplify request_frame loop.
lavfi/f_ebur128: remove request_frame hack.
lavfi/af_asetnsamples: simplify request_frame loop.
lavfi/af_atempo: simplify request_frame loop.
lavc: do not init frame with guessed layout.

It is breaking support from files with unknown layout.
Merge remote-tracking branch 'cigaes/master'

* cigaes/master:
  lavc: do not init frame with guessed layout.
  lavfi/af_atempo: simplify request_frame loop.
  lavfi/af_asetnsamples: simplify request_frame loop.
  lavfi/f_ebur128: remove request_frame hack.
  lavfi/vf_tile: simplify request_frame loop.
  lavfi: reindent after last commit.
  lavfi: loop on request_frame if necessary.

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