System: Add non-persistent memory card type

src/core/memory_card.h

@@ -21,6 +21,7 @@ public:
   static std::unique_ptr<MemoryCard> Open(std::string_view filename);
 
   const MemoryCardImage::DataArray& GetData() const { return m_data; }
+  MemoryCardImage::DataArray& GetData() { return m_data; }
   const std::string& GetFilename() const { return m_filename; }
   void SetFilename(std::string filename) { m_filename = std::move(filename); }