Disable FastAlloc for 64 bit builds

FastAlloc only uses the lower 32 bits of a pointer to index its internal
structures which is a problem for allocations with size >= 0x8001,
because multiple big allocations can have the same values for bit 16 to
bit 32, which causes FastAlloc to discard its info about the previous
allocation

OBSApi/Utility/FastAlloc.cpp

@@ -245,6 +245,7 @@ void * __restrict FastAlloc::_Allocate(size_t dwSize)
         }
         pool = &poollist[(PtrTo32(lpMemory)>>16)&0xFF];
 
+        assert(pool->blocksUsed == 0)
         pool->blocksUsed = 1;
         pool->bytesTotal = dwSize;
         pool->lpMem = lpMemory;

OBSApi/Utility/XT.cpp

@@ -107,7 +107,11 @@ void STDCALL ResetXTAllocator(CTSTR lpAllocator)
     else if (scmpi(lpAllocator, TEXT("SeriousMemoryDebuggingAlloc")) == 0)
         MainAllocator = new SeriousMemoryDebuggingAlloc;
     else
+#if defined(_M_X64) || defined(__amd64__)
+        MainAllocator = new DefaultAlloc;
+#else
         MainAllocator = new FastAlloc;
+#endif
 
     locale = new LocaleStringLookup;