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/GBACart.cpp

@@ -100,6 +100,15 @@ void CartCommon::SRAMWrite(u32 addr, u8 val)
 {
 }
 
+u8* CartCommon::GetSaveMemory() const
+{
+    return nullptr;
+}
+
+u32 CartCommon::GetSaveMemoryLength() const
+{
+    return 0;
+}
 
 CartGame::CartGame(u8* rom, u32 len) : CartCommon()
 {
@@ -334,6 +343,16 @@ void CartGame::SRAMWrite(u32 addr, u8 val)
     }
 }
 
+u8* CartGame::GetSaveMemory() const
+{
+    return SRAM;
+}
+
+u32 CartGame::GetSaveMemoryLength() const
+{
+    return SRAMLength;
+}
+
 void CartGame::ProcessGPIO()
 {
 }
@@ -878,4 +897,14 @@ void SRAMWrite(u32 addr, u8 val)
     if (Cart) Cart->SRAMWrite(addr, val);
 }
 
+u8* GetSaveMemory()
+{
+    return Cart ? Cart->GetSaveMemory() : nullptr;
+}
+
+u32 GetSaveMemoryLength()
+{
+    return Cart ? Cart->GetSaveMemoryLength() : 0;
+}
+
 }