lavfi/buffersrc: set channel layout if it is known.

Introduced in 01649c7, lost in the API change.
lavfi/buffersrc: check channel count changes.
lavfi/buffersrc: disable deprecated warnings.
lavfi/buffersink: fix header.

Move non-obsolete functions, structures and constants outside
the scope of "#if FF_API_AVFILTERBUFFER".

Add attribute_deprecated to av_buffersink_get_buffer_ref()
and av_buffersink_poll_frame().

Add doxygen for av_buffersink_get_frame_flags().
Merge remote-tracking branch 'cigaes/master'

* cigaes/master:
  lavfi/buffersink: fix header.
  lavfi/buffersrc: disable deprecated warnings.
  lavfi/buffersrc: check channel count changes.
  lavfi/buffersrc: set channel layout if it is known.

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