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_metal.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 +20,10 @@
 */
 
 /**
- *  \file SDL_metal.h
+ * # CategoryMetal
  *
- *  Header file for functions to creating Metal layers and views on SDL windows.
+ * Header file for functions to creating Metal layers and views on SDL
+ * windows.
  */
 
 #ifndef SDL_metal_h_
@@ -37,9 +38,9 @@ extern "C" {
 #endif
 
 /**
- *  \brief A handle to a CAMetalLayer-backed NSView (macOS) or UIView (iOS/tvOS).
+ * A handle to a CAMetalLayer-backed NSView (macOS) or UIView (iOS/tvOS).
  *
- *  \note This can be cast directly to an NSView or UIView.
+ * This can be cast directly to an NSView or UIView.
  */
 typedef void *SDL_MetalView;
 
@@ -90,9 +91,9 @@ extern DECLSPEC void *SDLCALL SDL_Metal_GetLayer(SDL_MetalView view);
  * Get the size of a window's underlying drawable in pixels (for use with
  * setting viewport, scissor & etc).
  *
- * \param window SDL_Window from which the drawable size should be queried
- * \param w Pointer to variable for storing the width in pixels, may be NULL
- * \param h Pointer to variable for storing the height in pixels, may be NULL
+ * \param window SDL_Window from which the drawable size should be queried.
+ * \param w Pointer to variable for storing the width in pixels, may be NULL.
+ * \param h Pointer to variable for storing the height in pixels, may be NULL.
  *
  * \since This function is available since SDL 2.0.14.
  *