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.cpp

@@ -383,6 +383,16 @@ void CartCommon::SetIRQ()
     NDS::SetIRQ(1, NDS::IRQ_CartIREQMC);
 }
 
+u8 *CartCommon::GetSaveMemory() const
+{
+    return nullptr;
+}
+
+u32 CartCommon::GetSaveMemoryLength() const
+{
+    return 0;
+}
+
 void CartCommon::ReadROM(u32 addr, u32 len, u8* data, u32 offset)
 {
     if (addr >= ROMLength) return;
@@ -553,6 +563,16 @@ u8 CartRetail::SPIWrite(u8 val, u32 pos, bool last)
     }
 }
 
+u8 *CartRetail::GetSaveMemory() const
+{
+    return SRAM;
+}
+
+u32 CartRetail::GetSaveMemoryLength() const
+{
+    return SRAMLength;
+}
+
 void CartRetail::ReadROM_B7(u32 addr, u32 len, u8* data, u32 offset)
 {
     addr &= (ROMLength-1);
@@ -1732,6 +1752,16 @@ void SetupDirectBoot(std::string romname)
         Cart->SetupDirectBoot(romname);
 }
 
+u8* GetSaveMemory()
+{
+    return Cart ? Cart->GetSaveMemory() : nullptr;
+}
+
+u32 GetSaveMemoryLength()
+{
+    return Cart ? Cart->GetSaveMemoryLength() : 0;
+}
+
 void EjectCart()
 {
     if (!CartInserted) return;