Add support for Audible AA files

https://en.wikipedia.org/wiki/Audible.com#Quality

MAINTAINERS

@@ -399,6 +399,7 @@ Generic parts:
 
 Muxers/Demuxers:
   4xm.c                                 Mike Melanson
+  aadec.c                               Vesselin Bontchev (vesselin.bontchev at yandex dot com)
   adtsenc.c                             Robert Swain
   afc.c                                 Paul B Mahol
   aiffdec.c                             Baptiste Coudurier, Matthieu Bouron

doc/demuxers.texi

@@ -18,6 +18,12 @@ enabled demuxers.
 
 The description of some of the currently available demuxers follows.
 
+@section aa
+
+Audible Format 2, 3, and 4 demuxer.
+
+This demuxer is used to demux Audible Format 2, 3, and 4 (.aa) files.
+
 @section applehttp
 
 Apple HTTP Live Streaming demuxer.

doc/general.texi

@@ -228,6 +228,8 @@ library:
 @item 8088flex TMV              @tab   @tab X
 @item AAX                       @tab   @tab X
     @tab Audible Enhanced Audio format, used in audiobooks.
+@item AA                        @tab   @tab X
+    @tab Audible Format 2, 3, and 4, used in audiobooks.
 @item ACT Voice                 @tab   @tab X
     @tab contains G.729 audio
 @item Adobe Filmstrip           @tab X @tab X

libavformat/Makefile

@@ -59,6 +59,7 @@ OBJS-$(CONFIG_SHARED)                    += log2_tab.o golomb_tab.o
 
 # muxers/demuxers
 OBJS-$(CONFIG_A64_MUXER)                 += a64.o rawenc.o
+OBJS-$(CONFIG_AA_DEMUXER)                += aadec.o
 OBJS-$(CONFIG_AAC_DEMUXER)               += aacdec.o apetag.o img2.o rawdec.o
 OBJS-$(CONFIG_AC3_DEMUXER)               += ac3dec.o rawdec.o
 OBJS-$(CONFIG_AC3_MUXER)                 += rawenc.o

libavformat/allformats.c

@@ -60,6 +60,7 @@ void av_register_all(void)
 
     /* (de)muxers */
     REGISTER_MUXER   (A64,              a64);
+    REGISTER_DEMUXER (AA,               aa);
     REGISTER_DEMUXER (AAC,              aac);
     REGISTER_MUXDEMUX(AC3,              ac3);
     REGISTER_DEMUXER (ACT,              act);