This is a variant of showwaves. It is implemented as a different filter
so that the user is not allowed to use meaningless options which belong
to showwaves (such as rate).

Major edits done by Stefano Sabatini, from a patch by ubitux.

See thread:
From: Clément Bœsch <u@pkh.me>
To: ffmpeg-devel@ffmpeg.org
Date: Wed, 24 Dec 2014 15:03:26 +0100
Subject: [FFmpeg-devel] [PATCH] avfilter/showwaves: add single_pic option