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-2.0.16/SDL_loadso.h

@@ -0,0 +1,81 @@
+/*
+  Simple DirectMedia Layer
+  Copyright (C) 1997-2018 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
+  arising from the use of this software.
+
+  Permission is granted to anyone to use this software for any purpose,
+  including commercial applications, and to alter it and redistribute it
+  freely, subject to the following restrictions:
+
+  1. The origin of this software must not be misrepresented; you must not
+     claim that you wrote the original software. If you use this software
+     in a product, an acknowledgment in the product documentation would be
+     appreciated but is not required.
+  2. Altered source versions must be plainly marked as such, and must not be
+     misrepresented as being the original software.
+  3. This notice may not be removed or altered from any source distribution.
+*/
+
+/**
+ *  \file SDL_loadso.h
+ *
+ *  System dependent library loading routines
+ *
+ *  Some things to keep in mind:
+ *  \li These functions only work on C function names.  Other languages may
+ *      have name mangling and intrinsic language support that varies from
+ *      compiler to compiler.
+ *  \li 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.
+ *  \li Avoid namespace collisions.  If you load a symbol from the library,
+ *      it is not defined whether or not it goes into the global symbol
+ *      namespace for the application.  If it does and it conflicts with
+ *      symbols in your code or other shared libraries, you will not get
+ *      the results you expect. :)
+ */
+
+#ifndef SDL_loadso_h_
+#define SDL_loadso_h_
+
+#include "SDL_stdinc.h"
+#include "SDL_error.h"
+
+#include "begin_code.h"
+/* Set up for C function definitions, even when using C++ */
+#ifdef __cplusplus
+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.
+ */
+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().
+ */
+extern DECLSPEC void *SDLCALL SDL_LoadFunction(void *handle,
+                                               const char *name);
+
+/**
+ *  Unload a shared object from memory.
+ */
+extern DECLSPEC void SDLCALL SDL_UnloadObject(void *handle);
+
+/* Ends C function definitions when using C++ */
+#ifdef __cplusplus
+}
+#endif
+#include "close_code.h"
+
+#endif /* SDL_loadso_h_ */
+
+/* vi: set ts=4 sw=4 expandtab: */