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_bits.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_bits.h
+ * # CategoryBits
  *
- *  Functions for fiddling with bits and bitmasks.
+ * Functions for fiddling with bits and bitmasks.
  */
 
 #ifndef SDL_bits_h_
@@ -56,6 +56,12 @@ extern __inline int _SDL_bsr_watcom(Uint32);
     modify exact [eax] nomemory;
 #endif
 
+/**
+ * Use this function to get the index of the most significant (set) bit in a
+ *
+ * \param x the number to find the MSB of.
+ * \returns the index of the most significant bit of x, or -1 if x is 0.
+ */
 SDL_FORCE_INLINE int
 SDL_MostSignificantBitIndex32(Uint32 x)
 {