Enable GameInput v3 by default if it's available

GameInput is now the most functional Windows API for handling XInput controllers.

src/core/windows/SDL_gameinput.cpp

@@ -77,6 +77,11 @@ bool SDL_InitGameInput(IGameInput **ppGameInput)
     return true;
 }
 
+bool SDL_GameInputReady(void)
+{
+    return (g_pGameInput != NULL);
+}
+
 void SDL_QuitGameInput(void)
 {
     SDL_assert(g_nGameInputRefCount > 0);

src/core/windows/SDL_gameinput.h

@@ -49,6 +49,7 @@ using namespace GameInput::v1;
 #endif
 
 extern bool SDL_InitGameInput(IGameInput **ppGameInput);
+extern bool SDL_GameInputReady(void);
 extern void SDL_QuitGameInput(void);
 
 #endif // HAVE_GAMEINPUT_H