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-2.0.16/SDL_opengles2_gl2platform.h

@@ -0,0 +1,30 @@
+#ifndef __gl2platform_h_
+#define __gl2platform_h_
+
+/* $Revision: 10602 $ on $Date:: 2010-03-04 22:35:34 -0800 #$ */
+
+/*
+ * This document is licensed under the SGI Free Software B License Version
+ * 2.0. For details, see http://oss.sgi.com/projects/FreeB/ .
+ */
+
+/* Platform-specific types and definitions for OpenGL ES 2.X  gl2.h
+ *
+ * Adopters may modify khrplatform.h and this file to suit their platform.
+ * You are encouraged to submit all modifications to the Khronos group so that
+ * they can be included in future versions of this file.  Please submit changes
+ * by sending them to the public Khronos Bugzilla (http://khronos.org/bugzilla)
+ * by filing a bug against product "OpenGL-ES" component "Registry".
+ */
+
+/*#include <KHR/khrplatform.h>*/
+
+#ifndef GL_APICALL
+#define GL_APICALL  KHRONOS_APICALL
+#endif
+
+#ifndef GL_APIENTRY
+#define GL_APIENTRY KHRONOS_APIENTRY
+#endif
+
+#endif /* __gl2platform_h_ */