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_test_assert.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
@@ -66,7 +66,7 @@ void SDLTest_Assert(int assertCondition, SDL_PRINTF_FORMAT_STRING const char *as
  * \param assertCondition Evaluated condition or variable to assert; fail (==0) or pass (!=0).
  * \param assertDescription Message to log with the assert describing it.
  *
- * \returns Returns the assertCondition so it can be used to externally to break execution flow if desired.
+ * \returns the assertCondition so it can be used to externally to break execution flow if desired.
  */
 int SDLTest_AssertCheck(int assertCondition, SDL_PRINTF_FORMAT_STRING const char *assertDescription, ...) SDL_PRINTF_VARARG_FUNC(2);