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_misc.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
@@ -38,29 +38,33 @@ extern "C" {
 #endif
 
 /**
- * \brief Open an URL / URI in the browser or other
+ * Open a URL/URI in the browser or other appropriate external application.
  *
  * Open a URL in a separate, system-provided application. How this works will
- *  vary wildly depending on the platform. This will likely launch what
- *  makes sense to handle a specific URL's protocol (a web browser for http://,
- *  etc), but it might also be able to launch file managers for directories
- *  and other things.
+ * vary wildly depending on the platform. This will likely launch what makes
+ * sense to handle a specific URL's protocol (a web browser for `http://`,
+ * etc), but it might also be able to launch file managers for directories and
+ * other things.
  *
  * What happens when you open a URL varies wildly as well: your game window
- *  may lose focus (and may or may not lose focus if your game was fullscreen
- *  or grabbing input at the time). On mobile devices, your app will likely
- *  move to the background or your process might be paused. Any given platform
- *  may or may not handle a given URL.
+ * may lose focus (and may or may not lose focus if your game was fullscreen
+ * or grabbing input at the time). On mobile devices, your app will likely
+ * move to the background or your process might be paused. Any given platform
+ * may or may not handle a given URL.
  *
  * If this is unimplemented (or simply unavailable) for a platform, this will
- *  fail with an error. A successful result does not mean the URL loaded, just
- *  that we launched something to handle it (or at least believe we did).
+ * fail with an error. A successful result does not mean the URL loaded, just
+ * that we launched _something_ to handle it (or at least believe we did).
  *
  * All this to say: this function can be useful, but you should definitely
- *  test it on every platform you target.
+ * test it on every platform you target.
  *
- *   \param url A valid URL to open.
- *  \return 0 on success, or -1 on error.
+ * \param url A valid URL/URI to open. Use `file:///full/path/to/file` for
+ *            local files, if supported.
+ * \returns 0 on success, or -1 on error; call SDL_GetError() for more
+ *          information.
+ *
+ * \since This function is available since SDL 2.0.14.
  */
 extern DECLSPEC int SDLCALL SDL_OpenURL(const char *url);