Update to SDL 2.24.0 and add separate macOS UB2 dylib

Update mingw and macOS SDL libraries to 2.24.0. (MSVC .lib files and
Xcode project haven't been updated.)

There is now a separate SDL dylib for macOS Universal Binary 1 and 2
because SDL 2.24.0 dropped support for macOS 10.6 (which is last to
support x86). Documentation says SDL 2.24.0 also dropped macOS 10.7
and 10.8 too. So UB2 is now 10.9 or later while 10.5 to 10.8 live in
UB1.

Compiled using osxcross with macOS 11.3 SDK. Ran "strip -x" on Windows
and macOS libraries. Ad-hoc signed dylib using PyOxidizer rcodesign to
fix digest checksums after strip.

code/SDL2/include/SDL_loadso.h

@@ -1,6 +1,6 @@
 /*
   Simple DirectMedia Layer
-  Copyright (C) 1997-2018 Sam Lantinga <slouken@libsdl.org>
+  Copyright (C) 1997-2022 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
@@ -51,22 +51,56 @@ extern "C" {
 #endif
 
 /**
- *  This function dynamically loads a shared object and returns a pointer
- *  to the object handle (or NULL if there was an error).
- *  The 'sofile' parameter is a system dependent name of the object file.
+ * Dynamically load a shared object.
+ *
+ * \param sofile a system-dependent name of the object file
+ * \returns an opaque pointer to the object handle or NULL if there was an
+ *          error; call SDL_GetError() for more information.
+ *
+ * \since This function is available since SDL 2.0.0.
+ *
+ * \sa SDL_LoadFunction
+ * \sa SDL_UnloadObject
  */
 extern DECLSPEC void *SDLCALL SDL_LoadObject(const char *sofile);
 
 /**
- *  Given an object handle, this function looks up the address of the
- *  named function in the shared object and returns it.  This address
- *  is no longer valid after calling SDL_UnloadObject().
+ * Look up the address of the named function in a shared object.
+ *
+ * This function pointer is no longer valid after calling SDL_UnloadObject().
+ *
+ * This function can only look up C function names. Other languages may have
+ * name mangling and intrinsic language support that varies from compiler to
+ * compiler.
+ *
+ * Make sure you declare your function pointers with the same calling
+ * convention as the actual library function. Your code will crash
+ * mysteriously if you do not do this.
+ *
+ * If the requested function doesn't exist, NULL is returned.
+ *
+ * \param handle a valid shared object handle returned by SDL_LoadObject()
+ * \param name the name of the function to look up
+ * \returns a pointer to the function or NULL if there was an error; call
+ *          SDL_GetError() for more information.
+ *
+ * \since This function is available since SDL 2.0.0.
+ *
+ * \sa SDL_LoadObject
+ * \sa SDL_UnloadObject
  */
 extern DECLSPEC void *SDLCALL SDL_LoadFunction(void *handle,
                                                const char *name);
 
 /**
- *  Unload a shared object from memory.
+ * Unload a shared object from memory.
+ *
+ * \param handle a valid shared object handle returned by SDL_LoadObject()
+ *
+ * \since This function is available since SDL 2.0.0.
+ *
+ * \sa SDL_LoadFunction
+ * \sa SDL_LoadObject
  */
 extern DECLSPEC void SDLCALL SDL_UnloadObject(void *handle);