Update SDL to version 2.0.14.

libs/SDL2/include/SDL_test_common.h

@@ -1,6 +1,6 @@
 /*
   Simple DirectMedia Layer
-  Copyright (C) 1997-2019 Sam Lantinga <slouken@libsdl.org>
+  Copyright (C) 1997-2020 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
@@ -155,6 +155,19 @@ int SDLTest_CommonArg(SDLTest_CommonState * state, int index);
  */
 void SDLTest_CommonLogUsage(SDLTest_CommonState * state, const char *argv0, const char **options);
 
+/**
+ * \brief Returns common usage information
+ *
+ * You should (probably) be using SDLTest_CommonLogUsage() instead, but this
+ *  function remains for binary compatibility. Strings returned from this
+ *  function are valid until SDLTest_CommonQuit() is called, in which case
+ *  those strings' memory is freed and can no longer be used.
+ *
+ * \param state The common state describing the test window to create.
+ * \returns String with usage information
+ */
+const char *SDLTest_CommonUsage(SDLTest_CommonState * state);
+
 /**
  * \brief Open test window.
  *