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_error.h

@@ -1,6 +1,6 @@
 /*
   Simple DirectMedia Layer
-  Copyright (C) 1997-2020 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
@@ -40,41 +40,92 @@ extern "C" {
 
 
 /**
- *  \brief Set the error message for the current thread
+ * Set the SDL error message for the current thread.
  *
- *  \return -1, there is no error handling for this function
+ * Calling this function will replace any previous error message that was set.
+ *
+ * This function always returns -1, since SDL frequently uses -1 to signify an
+ * failing result, leading to this idiom:
+ *
+ * ```c
+ * if (error_code) {
+ *     return SDL_SetError("This operation has failed: %d", error_code);
+ * }
+ * ```
+ *
+ * \param fmt a printf()-style message format string
+ * \param ... additional parameters matching % tokens in the `fmt` string, if
+ *            any
+ * \returns always -1.
+ *
+ * \since This function is available since SDL 2.0.0.
+ *
+ * \sa SDL_ClearError
+ * \sa SDL_GetError
  */
 extern DECLSPEC int SDLCALL SDL_SetError(SDL_PRINTF_FORMAT_STRING const char *fmt, ...) SDL_PRINTF_VARARG_FUNC(1);
 
 /**
- *  \brief Get the last error message that was set
+ * Retrieve a message about the last error that occurred on the current
+ * thread.
  *
- * SDL API functions may set error messages and then succeed, so you should
- * only use the error value if a function fails.
- * 
- * This returns a pointer to a static buffer for convenience and should not
- * be called by multiple threads simultaneously.
+ * It is possible for multiple errors to occur before calling SDL_GetError().
+ * Only the last error is returned.
  *
- *  \return a pointer to the last error message that was set
+ * The message is only applicable when an SDL function has signaled an error.
+ * You must check the return values of SDL function calls to determine when to
+ * appropriately call SDL_GetError(). You should *not* use the results of
+ * SDL_GetError() to decide if an error has occurred! Sometimes SDL will set
+ * an error string even when reporting success.
+ *
+ * SDL will *not* clear the error string for successful API calls. You *must*
+ * check return values for failure cases before you can assume the error
+ * string applies.
+ *
+ * Error strings are set per-thread, so an error set in a different thread
+ * will not interfere with the current thread's operation.
+ *
+ * The returned string is internally allocated and must not be freed by the
+ * application.
+ *
+ * \returns a message with information about the specific error that occurred,
+ *          or an empty string if there hasn't been an error message set since
+ *          the last call to SDL_ClearError(). The message is only applicable
+ *          when an SDL function has signaled an error. You must check the
+ *          return values of SDL function calls to determine when to
+ *          appropriately call SDL_GetError().
+ *
+ * \since This function is available since SDL 2.0.0.
+ *
+ * \sa SDL_ClearError
+ * \sa SDL_SetError
  */
 extern DECLSPEC const char *SDLCALL SDL_GetError(void);
 
 /**
- *  \brief Get the last error message that was set for the current thread
+ * Get the last error message that was set for the current thread.
  *
- * SDL API functions may set error messages and then succeed, so you should
- * only use the error value if a function fails.
- * 
- *  \param errstr A buffer to fill with the last error message that was set
- *                for the current thread
- *  \param maxlen The size of the buffer pointed to by the errstr parameter
+ * This allows the caller to copy the error string into a provided buffer, but
+ * otherwise operates exactly the same as SDL_GetError().
  *
- *  \return errstr
+ * \param errstr A buffer to fill with the last error message that was set for
+ *               the current thread
+ * \param maxlen The size of the buffer pointed to by the errstr parameter
+ * \returns the pointer passed in as the `errstr` parameter.
+ *
+ * \since This function is available since SDL 2.0.14.
+ *
+ * \sa SDL_GetError
  */
 extern DECLSPEC char * SDLCALL SDL_GetErrorMsg(char *errstr, int maxlen);
 
 /**
- *  \brief Clear the error message for the current thread
+ * Clear any previous error message for this thread.
+ *
+ * \since This function is available since SDL 2.0.0.
+ *
+ * \sa SDL_GetError
+ * \sa SDL_SetError
  */
 extern DECLSPEC void SDLCALL SDL_ClearError(void);