lavfi: add audio mix filter

Changelog

@@ -20,6 +20,7 @@ version <next>:
 - audio filters support in libavfilter and avconv
 - add fps filter
 - audio split filter
+- audio mix filter
 
 
 version 0.8:

doc/filters.texi

@@ -133,6 +133,44 @@ For example to force the output to either unsigned 8-bit or signed 16-bit stereo
 aformat=sample_fmts\=u8\,s16:channel_layouts\=stereo
 @end example
 
+@section amix
+
+Mixes multiple audio inputs into a single output.
+
+For example
+@example
+avconv -i INPUT1 -i INPUT2 -i INPUT3 -filter_complex amix=inputs=3:duration=first:dropout_transition=3 OUTPUT
+@end example
+will mix 3 input audio streams to a single output with the same duration as the
+first input and a dropout transition time of 3 seconds.
+
+The filter accepts the following named parameters:
+@table @option
+
+@item inputs
+Number of inputs. If unspecified, it defaults to 2.
+
+@item duration
+How to determine the end-of-stream.
+@table @option
+
+@item longest
+Duration of longest input. (default)
+
+@item shortest
+Duration of shortest input.
+
+@item first
+Duration of first input.
+
+@end table
+
+@item dropout_transition
+Transition time, in seconds, for volume renormalization when an input
+stream ends. The default value is 2 seconds.
+
+@end table
+
 @section anull
 
 Pass the audio source unchanged to the output.

libavfilter/Makefile

@@ -25,6 +25,7 @@ OBJS = allfilters.o                                                     \
        video.o                                                          \
 
 OBJS-$(CONFIG_AFORMAT_FILTER)                += af_aformat.o
+OBJS-$(CONFIG_AMIX_FILTER)                   += af_amix.o
 OBJS-$(CONFIG_ANULL_FILTER)                  += af_anull.o
 OBJS-$(CONFIG_ASPLIT_FILTER)                 += split.o
 OBJS-$(CONFIG_ASYNCTS_FILTER)                += af_asyncts.o

libavfilter/allfilters.c

@@ -35,6 +35,7 @@ void avfilter_register_all(void)
     initialized = 1;
 
     REGISTER_FILTER (AFORMAT,     aformat,     af);
+    REGISTER_FILTER (AMIX,        amix,        af);
     REGISTER_FILTER (ANULL,       anull,       af);
     REGISTER_FILTER (ASPLIT,      asplit,      af);
     REGISTER_FILTER (ASYNCTS,     asyncts,     af);

libavfilter/version.h

@@ -29,7 +29,7 @@
 #include "libavutil/avutil.h"
 
 #define LIBAVFILTER_VERSION_MAJOR  2
-#define LIBAVFILTER_VERSION_MINOR  19
+#define LIBAVFILTER_VERSION_MINOR  20
 #define LIBAVFILTER_VERSION_MICRO  0
 
 #define LIBAVFILTER_VERSION_INT AV_VERSION_INT(LIBAVFILTER_VERSION_MAJOR, \