Add SDL_TryLockMutex and implementations for all platforms

include/SDL_mutex.h

@@ -73,6 +73,13 @@ extern DECLSPEC SDL_mutex *SDLCALL SDL_CreateMutex(void);
 #define SDL_LockMutex(m)	SDL_mutexP(m)
 extern DECLSPEC int SDLCALL SDL_mutexP(SDL_mutex * mutex);
 
+/**
+ *  Try to lock the mutex
+ *  
+ *  \return 0, SDL_MUTEX_TIMEDOUT, or -1 on error
+ */
+extern DECLSPEC int SDLCALL SDL_TryLockMutex(SDL_mutex * mutex);
+
 /**
  *  Unlock the mutex.
  *