Renamed SDL_ThreadID() to SDL_GetCurrentThreadID()

Also renamed SDL_threadID to SDL_ThreadID and made it Uint64 for consistency with other ID types

test/testerror.c

@@ -35,8 +35,8 @@ static int SDLCALL
 ThreadFunc(void *data)
 {
     /* Set the child thread error string */
-    SDL_SetError("Thread %s (%lu) had a problem: %s",
-                 (char *)data, SDL_ThreadID(), "nevermind");
+    SDL_SetError("Thread %s (%" SDL_PRIu64 ") had a problem: %s",
+                 (char *)data, SDL_GetCurrentThreadID(), "nevermind");
     while (alive) {
         SDL_Log("Thread '%s' is alive!\n", (char *)data);
         SDL_Delay(1 * 1000);