[Opus] Update to v1.2.1

third_party/opus/src/README

 == Opus audio codec ==
 
 Opus is a codec for interactive speech and audio transmission over the Internet.
 
   Opus can handle a wide range of interactive audio applications, including
 Voice over IP, videoconferencing, in-game  chat, and even remote live music
 performances. It can scale from low bit-rate narrowband speech to very high
 quality stereo music.
 
   Opus, when coupled with an appropriate container format, is also suitable
@@ skipping 25 matching lines @@
 
 Opus-tools can be found at:
  https://git.xiph.org/?p=opus-tools.git
 or on the main Opus website:
  https://opus-codec.org/
 
 == Compiling libopus ==
 
 To build from a distribution tarball, you only need to do the following:
 
-% ./configure
-% make
+    % ./configure
+    % make
 
 To build from the git repository, the following steps are necessary:
 
+0) Set up a development environment:
+
+On an Ubuntu or Debian family Linux distribution:
+
+    % sudo apt-get install git autoconf automake libtool gcc make
+
+On a Fedora/Redhat based Linux:
+
+    % sudo dnf install git autoconf automake libtool gcc make
+
+Or for older Redhat/Centos Linux releases:
+
+    % sudo yum install git autoconf automake libtool gcc make
+
+On Apple macOS, install Xcode and brew.sh, then in the Terminal enter:
+
+    % brew install autoconf automake libtool
+
 1) Clone the repository:
 
-% git clone https://git.xiph.org/opus.git
-% cd opus
+    % git clone https://git.xiph.org/opus.git
+    % cd opus
 
 2) Compiling the source
 
-% ./autogen.sh
-% ./configure
-% make
+    % ./autogen.sh
+    % ./configure
+    % make
 
 3) Install the codec libraries (optional)
 
-% sudo make install
+    % sudo make install
 
 Once you have compiled the codec, there will be a opus_demo executable
 in the top directory.
 
 Usage: opus_demo [-e] <application> <sampling rate (Hz)> <channels (1/2)>
          <bits per second> [options] <input> <output>
        opus_demo -d <sampling rate (Hz)> <channels (1/2)> [options]
          <input> <output>
 
 mode: voip | audio | restricted-lowdelay
@@ skipping 20 matching lines @@
 input and output are little-endian signed 16-bit PCM files or opus
 bitstreams with simple opus_demo proprietary framing.
 
 == Testing ==
 
 This package includes a collection of automated unit and system tests
 which SHOULD be run after compiling the package especially the first
 time it is run on a new platform.
 
 To run the integrated tests:
-% make check
+
+    % make check
 
 There is also collection of standard test vectors which are not
 included in this package for size reasons but can be obtained from:
 https://opus-codec.org/testvectors/opus_testvectors.tar.gz
 
 To run compare the code to these test vectors:
 
-% curl -O https://opus-codec.org/testvectors/opus_testvectors.tar.gz
-% tar -zxf opus_testvectors.tar.gz
-% ./tests/run_vectors.sh ./ opus_testvectors 48000
+    % curl -OL https://opus-codec.org/testvectors/opus_testvectors.tar.gz
+    % tar -zxf opus_testvectors.tar.gz
+    % ./tests/run_vectors.sh ./ opus_testvectors 48000
 
 == Portability notes ==
 
 This implementation uses floating-point by default but can be compiled to
 use only fixed-point arithmetic by setting --enable-fixed-point (if using
 autoconf) or by defining the FIXED_POINT macro (if building manually).
 The fixed point implementation has somewhat lower audio quality and is
 slower on platforms with fast FPUs, it is normally only used in embedded
 environments.
 
 The implementation can be compiled with either a C89 or a C99 compiler.
 While it does not rely on any _undefined behavior_ as defined by C89 or
 C99, it relies on common _implementation-defined behavior_ for two's
 complement architectures:
 
 o Right shifts of negative values are consistent with two's
   complement arithmetic, so that a>>b is equivalent to
   floor(a/(2^b)),
 
 o For conversion to a signed integer of N bits, the value is reduced
   modulo 2^N to be within range of the type,
 
 o The result of integer division of a negative value is truncated
   towards zero, and
 
 o The compiler provides a 64-bit integer type (a C99 requirement
   which is supported by most C89 compilers).