Update SDL2 to newest version to fix 7.1 playback

This fixes 7.1 playback. Before this commit, SDL used 7.1 front audio configuration with Center Left and Center Right speakers, but without Rear speakers.

See https://github.com/libsdl-org/SDL/issues/11709 for more details.

libs/SDL2/include/SDL_filesystem.h

@@ -1,6 +1,6 @@
 /*
   Simple DirectMedia Layer
-  Copyright (C) 1997-2023 Sam Lantinga <slouken@libsdl.org>
+  Copyright (C) 1997-2024 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
@@ -20,9 +20,9 @@
 */
 
 /**
- *  \file SDL_filesystem.h
+ * # CategoryFilesystem
  *
- *  \brief Include file for filesystem SDL API functions
+ * Include file for filesystem SDL API functions
  */
 
 #ifndef SDL_filesystem_h_
@@ -64,7 +64,7 @@ extern "C" {
  * directory of the application as it is uncommon to store resources outside
  * the executable. As such it is not a writable directory.
  *
- * The returned path is guaranteed to end with a path separator ('\' on
+ * The returned path is guaranteed to end with a path separator ('\\' on
  * Windows, '/' on most other platforms).
  *
  * The pointer returned is owned by the caller. Please call SDL_free() on the
@@ -120,14 +120,14 @@ extern DECLSPEC char *SDLCALL SDL_GetBasePath(void);
  * - ...only use letters, numbers, and spaces. Avoid punctuation like "Game
  *   Name 2: Bad Guy's Revenge!" ... "Game Name 2" is sufficient.
  *
- * The returned path is guaranteed to end with a path separator ('\' on
+ * The returned path is guaranteed to end with a path separator ('\\' on
  * Windows, '/' on most other platforms).
  *
  * The pointer returned is owned by the caller. Please call SDL_free() on the
  * pointer when done with it.
  *
- * \param org the name of your organization
- * \param app the name of your application
+ * \param org the name of your organization.
+ * \param app the name of your application.
  * \returns a UTF-8 string of the user directory in platform-dependent
  *          notation. NULL if there's a problem (creating directory failed,
  *          etc.).