WinRT: bug-fix - SDL_SetThreadPriority() didn't work on WinRT 8.x platforms

WinRT 8.0 (Phone and non-Phone) didn't offer an API to set an already-created
thread's priority.  WinRT 8.1 offered this API, along with several other
Win32 thread functions that were previously unavailable (in WinRT).

This change makes WinRT 8.1+ platforms use SDL's Win32 backend.

src/thread/windows/SDL_sysmutex.c

@@ -45,7 +45,11 @@ SDL_CreateMutex(void)
     if (mutex) {
         /* Initialize */
         /* On SMP systems, a non-zero spin count generally helps performance */
+#if __WINRT__
+        InitializeCriticalSectionEx(&mutex->cs, 2000, 0);
+#else
         InitializeCriticalSectionAndSpinCount(&mutex->cs, 2000);
+#endif
     } else {
         SDL_OutOfMemory();
     }