fix: 修复 D3D12SwapChain 重复创建 swapchain 问题

- 添加 Initialize(IDXGISwapChain*, width, height) 重载方法
- 接受已存在的 swapchain 而不是重复创建
- 测试通过，日志显示正常渲染

engine/include/XCEngine/RHI/D3D12/D3D12SwapChain.h

@@ -18,6 +18,7 @@ public:
     ~D3D12SwapChain();
 
     bool Initialize(IDXGIFactory4* factory, ID3D12CommandQueue* commandQueue, HWND windowHandle, uint32_t width, uint32_t height, uint32_t bufferCount = 2);
+    bool Initialize(IDXGISwapChain* swapChain, uint32_t width, uint32_t height);
     void Shutdown();
 
     uint32_t GetCurrentBackBufferIndex() const;