Update SDL2 to version 2.28.1.

libs/SDL2/include/SDL_surface.h

@@ -1,6 +1,6 @@
 /*
   Simple DirectMedia Layer
-  Copyright (C) 1997-2022 Sam Lantinga <slouken@libsdl.org>
+  Copyright (C) 1997-2023 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
@@ -61,6 +61,8 @@ extern "C" {
  */
 #define SDL_MUSTLOCK(S) (((S)->flags & SDL_RLEACCEL) != 0)
 
+typedef struct SDL_BlitMap SDL_BlitMap;  /* this is an opaque type. */
+
 /**
  * \brief A collection of pixels used in software blitting.
  *
@@ -88,7 +90,7 @@ typedef struct SDL_Surface
     SDL_Rect clip_rect;         /**< Read-only */
 
     /** info for fast blit mapping to other surfaces */
-    struct SDL_BlitMap *map;    /**< Private */
+    SDL_BlitMap *map;           /**< Private */
 
     /** Reference count -- used when freeing surface */
     int refcount;               /**< Read-mostly */