make virtual buffer become an optional

src/common/host_memory.cpp

@@ -690,7 +690,7 @@ private:
 HostMemory::HostMemory(size_t backing_size_, size_t virtual_size_) : backing_size(backing_size_), virtual_size(virtual_size_) {
 #ifdef __OPENORBIS__
     LOG_WARNING(HW_Memory, "Platform doesn't support fastmem");
-    fallback_buffer = std::make_unique<Common::VirtualBuffer<u8>>(backing_size);
+    fallback_buffer.emplace(backing_size);
     backing_base = fallback_buffer->data();
     virtual_base = nullptr;
 #else

src/common/host_memory.h

@@ -7,6 +7,7 @@
 #pragma once
 
 #include <memory>
+#include <optional>
 #include "common/common_funcs.h"
 #include "common/common_types.h"
 #include "common/virtual_buffer.h"