Re-added WinRT support until we're sure that it's no longer being used

src/thread/windows/SDL_sysmutex.c

@@ -44,12 +44,19 @@ SDL_mutex_impl_t SDL_mutex_impl_active = {0};
  * Implementation based on Slim Reader/Writer (SRW) Locks for Win 7 and newer.
  */
 
+#if __WINRT__
+/* Functions are guaranteed to be available */
+#define pReleaseSRWLockExclusive ReleaseSRWLockExclusive
+#define pAcquireSRWLockExclusive AcquireSRWLockExclusive
+#define pTryAcquireSRWLockExclusive TryAcquireSRWLockExclusive
+#else
 typedef VOID(WINAPI *pfnReleaseSRWLockExclusive)(PSRWLOCK);
 typedef VOID(WINAPI *pfnAcquireSRWLockExclusive)(PSRWLOCK);
 typedef BOOLEAN(WINAPI *pfnTryAcquireSRWLockExclusive)(PSRWLOCK);
 static pfnReleaseSRWLockExclusive pReleaseSRWLockExclusive = NULL;
 static pfnAcquireSRWLockExclusive pAcquireSRWLockExclusive = NULL;
 static pfnTryAcquireSRWLockExclusive pTryAcquireSRWLockExclusive = NULL;
+#endif
 
 static SDL_mutex *
 SDL_CreateMutex_srw(void)
@@ -173,7 +180,11 @@ SDL_CreateMutex_cs(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();
     }
@@ -256,6 +267,10 @@ SDL_CreateMutex(void)
         const SDL_mutex_impl_t * impl = &SDL_mutex_impl_cs;
 
         if (!SDL_GetHintBoolean(SDL_HINT_WINDOWS_FORCE_MUTEX_CRITICAL_SECTIONS, SDL_FALSE)) {
+#if __WINRT__
+            /* Link statically on this platform */
+            impl = &SDL_mutex_impl_srw;
+#else
             /* Try faster implementation for Windows 7 and newer */
             HMODULE kernel32 = GetModuleHandle(TEXT("kernel32.dll"));
             if (kernel32) {
@@ -268,6 +283,7 @@ SDL_CreateMutex(void)
                     impl = &SDL_mutex_impl_srw;
                 }
             }
+#endif
         }
 
         /* Copy instead of using pointer to save one level of indirection */