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_harness.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
@@ -76,9 +76,9 @@ typedef struct SDLTest_TestCaseReference {
     /* !< Func2Stress */
     SDLTest_TestCaseFp testCase;
     /* !< Short name (or function name) "Func2Stress" */
-    char *name;
+    const char *name;
     /* !< Long name or full description "This test pushes func2() to the limit." */
-    char *description;
+    const char *description;
     /* !< Set to TEST_ENABLED or TEST_DISABLED (test won't be run) */
     int enabled;
 } SDLTest_TestCaseReference;
@@ -88,7 +88,7 @@ typedef struct SDLTest_TestCaseReference {
  */
 typedef struct SDLTest_TestSuiteReference {
     /* !< "PlatformSuite" */
-    char *name;
+    const char *name;
     /* !< The function that is run before each test. NULL skips. */
     SDLTest_TestCaseSetUpFp testSetUp;
     /* !< The test cases that are run as part of the suite. Last item should be NULL. */
@@ -105,7 +105,7 @@ typedef struct SDLTest_TestSuiteReference {
  *
  * \param length The length of the seed string to generate
  *
- * \returns The generated seed string
+ * \returns the generated seed string
  */
 char *SDLTest_GenerateRunSeed(const int length);
 
@@ -118,7 +118,7 @@ char *SDLTest_GenerateRunSeed(const int length);
  * \param filter Filter specification. NULL disables. Case sensitive.
  * \param testIterations Number of iterations to run each test case.
  *
- * \returns Test run result; 0 when all tests passed, 1 if any tests failed.
+ * \returns the test run result: 0 when all tests passed, 1 if any tests failed.
  */
 int SDLTest_RunSuites(SDLTest_TestSuiteReference *testSuites[], const char *userRunSeed, Uint64 userExecKey, const char *filter, int testIterations);