Update SDL Mixer to 2.0.4; remove unnecessary extra lib and include folders

libs/SDL2_mixer/CHANGES.txt

+2.0.4:
+Ozkan Sezer - Wed, 10 Oct 2018 14:56:10
+ * Removed smpeg support for mp3 music, now that it's replaced by libmpg123
+Ozkan Sezer - Sun, 07 Oct 2018 08:50:02
+ * Fixed mp3 mad decoder to skip tags, which otherwise would lead to crashes
+Ozkan Sezer - Fri, 15 Jun 2018 05:32:56
+ * Added support for Opus music playback using opusfile library
+
+2.0.3:
+Sam Lantinga - Thu, Mar  1, 2018  9:06:58 AM
+ * Fixed regression where Mix_Init() would return 0 for available music formats
+
 2.0.2:
 Sam Lantinga - Fri Oct 20 22:04:50 PDT 2017
  * Implemented 24-bit and surround sound support for FLAC audio files

libs/SDL2_mixer/COPYING.txt

 /*
   SDL_mixer:  An audio mixer library based on the SDL library
-  Copyright (C) 1997-2017 Sam Lantinga <slouken@libsdl.org>
+  Copyright (C) 1997-2018 Sam Lantinga <slouken@libsdl.org>
 
   This software is provided 'as-is', without any express or implied
   warranty.  In no event will the authors be held liable for any damages

libs/SDL2_mixer/README.txt

@@ -14,7 +14,7 @@ for documentation on this mixer library.
 The mixer can currently load Microsoft WAVE files and Creative Labs VOC
 files as audio samples, it can load FLAC files with libFLAC, it can load
 Ogg Vorbis files with Ogg Vorbis or Tremor libraries, it can load MP3 files
-using mpg123, SMPEG or libmad, and it can load MIDI files with Timidity,
+using mpg123 or libmad, and it can load MIDI files with Timidity,
 FluidSynth, and natively on Windows, Mac OSX, and Linux, and finally it can
 load the following file formats via ModPlug or MikMod: .MOD .S3M .IT .XM.
 

libs/SDL2_mixer/i686-w64-mingw32/bin/LICENSE.FLAC.txt

 The source code to this library used with SDL_mixer can be found here:
-http://www.libsdl.org/projects/SDL_mixer/libs/
+https://hg.libsdl.org/SDL_mixer/file/default/external
 ---
 
 Copyright (C) 2000,2001,2002,2003,2004,2005,2006,2007  Josh Coalson

libs/SDL2_mixer/i686-w64-mingw32/bin/LICENSE.modplug.txt

+The source code to this library used with SDL_mixer can be found here:
+https://hg.libsdl.org/SDL_mixer/file/default/external
+---
+
 ModPlug-XMMS and libmodplug are now in the public domain.

libs/SDL2_mixer/i686-w64-mingw32/bin/LICENSE.mpg123.txt

+The source code to this library used with SDL_mixer can be found here:
+https://hg.libsdl.org/SDL_mixer/file/default/external
+---
+
 This is the file that contains the terms of use, copying, etc. for the mpg123 distribution package.
 
 Main message, to include in "About ..." boxes, etc:

libs/SDL2_mixer/i686-w64-mingw32/bin/LICENSE.ogg-vorbis.txt

 The source code to this library used with SDL_mixer can be found here:
-http://www.libsdl.org/projects/SDL_mixer/libs/
+https://hg.libsdl.org/SDL_image/file/default/external
 ---
 
 Copyright (c) 2002-2008 Xiph.org Foundation

libs/SDL2_mixer/lib/x64/LICENSE.FLAC.txt → libs/SDL2_mixer/i686-w64-mingw32/bin/LICENSE.opus.txt

 The source code to this library used with SDL_mixer can be found here:
-http://www.libsdl.org/projects/SDL_mixer/libs/
+https://hg.libsdl.org/SDL_mixer/file/default/external
 ---
 
-Copyright (C) 2000,2001,2002,2003,2004,2005,2006,2007  Josh Coalson
+Copyright 2001-2011 Xiph.Org, Skype Limited, Octasic,
+                    Jean-Marc Valin, Timothy B. Terriberry,
+                    CSIRO, Gregory Maxwell, Mark Borgerding,
+                    Erik de Castro Lopo
 
 Redistribution and use in source and binary forms, with or without
 modification, are permitted provided that the following conditions
@@ -15,18 +18,31 @@ notice, this list of conditions and the following disclaimer.
 notice, this list of conditions and the following disclaimer in the
 documentation and/or other materials provided with the distribution.
 
-- Neither the name of the Xiph.org Foundation nor the names of its
-contributors may be used to endorse or promote products derived from
-this software without specific prior written permission.
+- Neither the name of Internet Society, IETF or IETF Trust, nor the 
+names of specific contributors, may be used to endorse or promote
+products derived from this software without specific prior written
+permission.
 
 THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
 ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
 LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
-A PARTICULAR PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE FOUNDATION OR
-CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
+A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER
+OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
 EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
 PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
 PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF
 LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING
 NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
 SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+Opus is subject to the royalty-free patent licenses which are
+specified at:
+
+Xiph.Org Foundation:
+https://datatracker.ietf.org/ipr/1524/
+
+Microsoft Corporation:
+https://datatracker.ietf.org/ipr/1914/
+
+Broadcom Corporation:
+https://datatracker.ietf.org/ipr/1526/

libs/SDL2_mixer/lib/x64/LICENSE.ogg-vorbis.txt → libs/SDL2_mixer/i686-w64-mingw32/bin/LICENSE.opusfile.txt

 The source code to this library used with SDL_mixer can be found here:
-http://www.libsdl.org/projects/SDL_mixer/libs/
+https://hg.libsdl.org/SDL_mixer/file/default/external
 ---
 
-Copyright (c) 2002-2008 Xiph.org Foundation
+Copyright (c) 1994-2013 Xiph.Org Foundation and contributors
 
 Redistribution and use in source and binary forms, with or without
 modification, are permitted provided that the following conditions
@@ -15,7 +15,7 @@ notice, this list of conditions and the following disclaimer.
 notice, this list of conditions and the following disclaimer in the
 documentation and/or other materials provided with the distribution.
 
-- Neither the name of the Xiph.org Foundation nor the names of its
+- Neither the name of the Xiph.Org Foundation nor the names of its
 contributors may be used to endorse or promote products derived from
 this software without specific prior written permission.
 

libs/SDL2_mixer/i686-w64-mingw32/include/SDL2/SDL_mixer.h

 /*
   SDL_mixer:  An audio mixer library based on the SDL library
-  Copyright (C) 1997-2017 Sam Lantinga <slouken@libsdl.org>
+  Copyright (C) 1997-2018 Sam Lantinga <slouken@libsdl.org>
 
   This software is provided 'as-is', without any express or implied
   warranty.  In no event will the authors be held liable for any damages
@@ -38,7 +38,7 @@ extern "C" {
 */
 #define SDL_MIXER_MAJOR_VERSION 2
 #define SDL_MIXER_MINOR_VERSION 0
-#define SDL_MIXER_PATCHLEVEL    2
+#define SDL_MIXER_PATCHLEVEL    4
 
 /* This macro can be used to fill a version structure with the compile-time
  * version of the SDL_mixer library.
@@ -80,7 +80,8 @@ typedef enum
     MIX_INIT_MOD    = 0x00000002,
     MIX_INIT_MP3    = 0x00000008,
     MIX_INIT_OGG    = 0x00000010,
-    MIX_INIT_MID    = 0x00000020
+    MIX_INIT_MID    = 0x00000020,
+    MIX_INIT_OPUS   = 0x00000040
 } MIX_InitFlags;
 
 /* Loads dynamic libraries and prepares them for use.  Flags should be
@@ -134,7 +135,8 @@ typedef enum {
     MUS_MP3,
     MUS_MP3_MAD_UNUSED,
     MUS_FLAC,
-    MUS_MODPLUG_UNUSED
+    MUS_MODPLUG_UNUSED,
+    MUS_OPUS
 } Mix_MusicType;
 
 /* The internal format for a music chunk interpreted via mikmod */