lavfi: add showwavespic filter

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

Changelog

@@ -5,6 +5,7 @@ version <next>:
 - FFT video filter
 - TDSC decoder
 - DTS lossless extension (XLL) decoding (not lossless, disabled by default)
+- showwavespic filter
 
 
 version 2.6:

doc/filters.texi

@@ -11741,6 +11741,33 @@ aevalsrc=sin(1*2*PI*t)*sin(880*2*PI*t):cos(2*PI*200*t),asplit[out0],showwaves=r=
 @end example
 @end itemize
 
+@section showwavespic
+
+Convert input audio to a single video frame, representing the samples waves.
+
+The filter accepts the following options:
+
+@table @option
+@item size, s
+Specify the video size for the output. For the syntax of this option, check the
+@ref{video size syntax,,"Video size" section in the ffmpeg-utils manual,ffmpeg-utils}.
+Default value is @code{600x240}.
+
+@item split_channels
+Set if channels should be drawn separately or overlap. Default value is 0.
+@end table
+
+@subsection Examples
+
+@itemize
+@item
+Extract a channel split representation of the wave form of a whole audio track
+in a 1024x800 picture using @command{ffmpeg}:
+@example
+ffmpeg -i audio.flac -lavfi showwavespic=split_channels=1:s=1024x800 waveform.png
+@end example
+@end itemize
+
 @section split, asplit
 
 Split input into several identical outputs.

libavfilter/Makefile

@@ -236,6 +236,7 @@ OBJS-$(CONFIG_CONCAT_FILTER)                 += avf_concat.o
 OBJS-$(CONFIG_SHOWCQT_FILTER)                += avf_showcqt.o
 OBJS-$(CONFIG_SHOWSPECTRUM_FILTER)           += avf_showspectrum.o
 OBJS-$(CONFIG_SHOWWAVES_FILTER)              += avf_showwaves.o
+OBJS-$(CONFIG_SHOWWAVESPIC_FILTER)           += avf_showwaves.o
 
 # multimedia sources
 OBJS-$(CONFIG_AMOVIE_FILTER)                 += src_movie.o

libavfilter/allfilters.c

@@ -251,6 +251,7 @@ void avfilter_register_all(void)
     REGISTER_FILTER(SHOWCQT,        showcqt,        avf);
     REGISTER_FILTER(SHOWSPECTRUM,   showspectrum,   avf);
     REGISTER_FILTER(SHOWWAVES,      showwaves,      avf);
+    REGISTER_FILTER(SHOWWAVESPIC,   showwavespic,   avf);
 
     /* multimedia sources */
     REGISTER_FILTER(AMOVIE,         amovie,         avsrc);

libavfilter/version.h

@@ -30,7 +30,7 @@
 #include "libavutil/version.h"
 
 #define LIBAVFILTER_VERSION_MAJOR  5
-#define LIBAVFILTER_VERSION_MINOR  12
+#define LIBAVFILTER_VERSION_MINOR  13
 #define LIBAVFILTER_VERSION_MICRO 100
 
 #define LIBAVFILTER_VERSION_INT AV_VERSION_INT(LIBAVFILTER_VERSION_MAJOR, \