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_power.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
@@ -37,7 +37,7 @@ extern "C" {
 #endif
 
 /**
- *  \brief The basic state for the system's power supply.
+ *  The basic state for the system's power supply.
  */
 typedef enum
 {
@@ -50,17 +50,30 @@ typedef enum
 
 
 /**
- *  \brief Get the current power supply details.
+ * Get the current power supply details.
  *
- *  \param secs Seconds of battery life left. You can pass a NULL here if
- *              you don't care. Will return -1 if we can't determine a
- *              value, or we're not running on a battery.
+ * You should never take a battery status as absolute truth. Batteries
+ * (especially failing batteries) are delicate hardware, and the values
+ * reported here are best estimates based on what that hardware reports. It's
+ * not uncommon for older batteries to lose stored power much faster than it
+ * reports, or completely drain when reporting it has 20 percent left, etc.
  *
- *  \param pct Percentage of battery life left, between 0 and 100. You can
- *             pass a NULL here if you don't care. Will return -1 if we
- *             can't determine a value, or we're not running on a battery.
+ * Battery status can change at any time; if you are concerned with power
+ * state, you should call this function frequently, and perhaps ignore changes
+ * until they seem to be stable for a few seconds.
  *
- *  \return The state of the battery (if any).
+ * It's possible a platform can only report battery percentage or time left
+ * but not both.
+ *
+ * \param secs seconds of battery life left, you can pass a NULL here if you
+ *             don't care, will return -1 if we can't determine a value, or
+ *             we're not running on a battery
+ * \param pct percentage of battery life left, between 0 and 100, you can pass
+ *            a NULL here if you don't care, will return -1 if we can't
+ *            determine a value, or we're not running on a battery
+ * \returns an SDL_PowerState enum representing the current battery state.
+ *
+ * \since This function is available since SDL 2.0.0.
  */
 extern DECLSPEC SDL_PowerState SDLCALL SDL_GetPowerInfo(int *secs, int *pct);