Expose SRAM pointers for frontends that manage save data their own way (#1643)

* Add a clarifying comment

- In case it saves some poor bastard hours of fruitless work

* Expose GBA and NDS save memory

- Add GetSaveMemory and GetSaveMemoryLength functions
- Where unsupported, they return null and zero

src/NDSCart.h

@@ -52,6 +52,9 @@ public:
 
     virtual u8 SPIWrite(u8 val, u32 pos, bool last);
 
+    virtual u8* GetSaveMemory() const;
+    virtual u32 GetSaveMemoryLength() const;
+
 protected:
     void ReadROM(u32 addr, u32 len, u8* data, u32 offset);
 
@@ -88,6 +91,9 @@ public:
 
     virtual u8 SPIWrite(u8 val, u32 pos, bool last) override;
 
+    virtual u8* GetSaveMemory() const override;
+    virtual u32 GetSaveMemoryLength() const override;
+
 protected:
     void ReadROM_B7(u32 addr, u32 len, u8* data, u32 offset);
 
@@ -223,6 +229,19 @@ bool LoadROM(const u8* romdata, u32 romlen);
 void LoadSave(const u8* savedata, u32 savelen);
 void SetupDirectBoot(std::string romname);
 
+/// This function is intended to allow frontends to save and load SRAM
+/// without using melonDS APIs.
+/// Modifying the emulated SRAM for any other reason is strongly discouraged.
+/// The returned pointer may be invalidated if the emulator is reset,
+/// or when a new game is loaded.
+/// Consequently, don't store the returned pointer for any longer than necessary.
+/// @returns Pointer to this cart's SRAM if a cart is loaded and supports SRAM, otherwise \c nullptr.
+u8* GetSaveMemory();
+
+/// @returns The length of the buffer returned by ::GetSaveMemory()
+/// if a cart is loaded and supports SRAM, otherwise zero.
+u32 GetSaveMemoryLength();
+
 void EjectCart();
 
 void ResetCart();