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();
     }

src/thread/windows/SDL_syssem.c

@@ -45,7 +45,11 @@ SDL_CreateSemaphore(Uint32 initial_value)
     sem = (SDL_sem *) SDL_malloc(sizeof(*sem));
     if (sem) {
         /* Create the semaphore, with max value 32K */
+#if __WINRT__
+        sem->id = CreateSemaphoreEx(NULL, initial_value, 32 * 1024, NULL, 0, SEMAPHORE_ALL_ACCESS);
+#else
         sem->id = CreateSemaphore(NULL, initial_value, 32 * 1024, NULL);
+#endif
         sem->count = initial_value;
         if (!sem->id) {
             SDL_SetError("Couldn't create semaphore");
@@ -86,7 +90,11 @@ SDL_SemWaitTimeout(SDL_sem * sem, Uint32 timeout)
     } else {
         dwMilliseconds = (DWORD) timeout;
     }
+#if __WINRT__
+    switch (WaitForSingleObjectEx(sem->id, dwMilliseconds, FALSE)) {
+#else
     switch (WaitForSingleObject(sem->id, dwMilliseconds)) {
+#endif
     case WAIT_OBJECT_0:
         InterlockedDecrement(&sem->count);
         retval = 0;

src/thread/windows/SDL_systhread.c

@@ -106,7 +106,7 @@ SDL_SYS_CreateThread(SDL_Thread * thread, void *args,
                      pfnSDL_CurrentBeginThread pfnBeginThread,
                      pfnSDL_CurrentEndThread pfnEndThread)
 {
-#elif defined(__CYGWIN__)
+#elif defined(__CYGWIN__) || defined(__WINRT__)
 int
 SDL_SYS_CreateThread(SDL_Thread * thread, void *args)
 {
@@ -230,7 +230,11 @@ SDL_SYS_SetThreadPriority(SDL_ThreadPriority priority)
 void
 SDL_SYS_WaitThread(SDL_Thread * thread)
 {
+#if __WINRT__
+    WaitForSingleObjectEx(thread->handle, INFINITE, FALSE);
+#else
     WaitForSingleObject(thread->handle, INFINITE);
+#endif
     CloseHandle(thread->handle);
 }