Note: the implementation could be more efficient, but at
the cost of more diff.

Most of the code from the following commit:

commit a2cd9be2
Author: Anton Khirnov <anton@khirnov.net>
Date:   Fri May 4 19:22:38 2012 +0200

    lavfi: add an audio buffer sink.

Adapted to call av_buffersink_get_frame_flags() instead of
accessing the frame directly.