Removed the need for SDL_CreateTLS()

This eliminates the tap dancing needed for allocating TLS slots, we'll automatically allocate them as needed, in a thread-safe way.

src/thread/SDL_thread_c.h

@@ -78,7 +78,7 @@ extern void SDL_RunThread(SDL_Thread *thread);
 /* This is the system-independent thread local storage structure */
 typedef struct
 {
-    SDL_TLSID limit;
+    int limit;
     struct
     {
         void *data;