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_clipboard.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
@@ -39,23 +39,46 @@ extern "C" {
 /* Function prototypes */
 
 /**
- * \brief Put UTF-8 text into the clipboard
+ * Put UTF-8 text into the clipboard.
  *
- * \sa SDL_GetClipboardText()
+ * \param text the text to store in the clipboard
+ * \returns 0 on success or a negative error code on failure; call
+ *          SDL_GetError() for more information.
+ *
+ * \since This function is available since SDL 2.0.0.
+ *
+ * \sa SDL_GetClipboardText
+ * \sa SDL_HasClipboardText
  */
 extern DECLSPEC int SDLCALL SDL_SetClipboardText(const char *text);
 
 /**
- * \brief Get UTF-8 text from the clipboard, which must be freed with SDL_free()
+ * Get UTF-8 text from the clipboard, which must be freed with SDL_free().
  *
- * \sa SDL_SetClipboardText()
+ * This functions returns empty string if there was not enough memory left for
+ * a copy of the clipboard's content.
+ *
+ * \returns the clipboard text on success or an empty string on failure; call
+ *          SDL_GetError() for more information. Caller must call SDL_free()
+ *          on the returned pointer when done with it (even if there was an
+ *          error).
+ *
+ * \since This function is available since SDL 2.0.0.
+ *
+ * \sa SDL_HasClipboardText
+ * \sa SDL_SetClipboardText
  */
 extern DECLSPEC char * SDLCALL SDL_GetClipboardText(void);
 
 /**
- * \brief Returns a flag indicating whether the clipboard exists and contains a text string that is non-empty
+ * Query whether the clipboard exists and contains a non-empty text string.
  *
- * \sa SDL_GetClipboardText()
+ * \returns SDL_TRUE if the clipboard has text, or SDL_FALSE if it does not.
+ *
+ * \since This function is available since SDL 2.0.0.
+ *
+ * \sa SDL_GetClipboardText
+ * \sa SDL_SetClipboardText
  */
 extern DECLSPEC SDL_bool SDLCALL SDL_HasClipboardText(void);