core: advance the source pointer for each backing run in TryWriteBacking (#5316)

Co-authored-by: Claude Sonnet <[email protected]>

src/core/memory.cpp

@@ -193,6 +193,7 @@ bool MemoryManager::TryWriteBacking(void* address, const void* data, u64 size) {
             u8* backing = impl.BackingBase() + phys_handle->second.base + start_in_dma;
             u64 copy_size = std::min<u64>(size, phys_handle->second.size - start_in_dma);
             memcpy(backing, data, copy_size);
+            data = static_cast<const u8*>(data) + copy_size;
             size -= copy_size;
         }
     }