VertexManagerBase: Make CreateNativeVertexFormat return a unique_ptr

Much safer as opposed to just returning raw allocated memory.

Source/Core/VideoBackends/D3D/NativeVertexFormat.cpp

@@ -27,10 +27,10 @@ private:
   ID3D11InputLayout* m_layout = nullptr;
 };
 
-NativeVertexFormat*
+std::unique_ptr<NativeVertexFormat>
 VertexManager::CreateNativeVertexFormat(const PortableVertexDeclaration& vtx_decl)
 {
-  return new D3DVertexFormat(vtx_decl);
+  return std::make_unique<D3DVertexFormat>(vtx_decl);
 }
 
 static const DXGI_FORMAT d3d_format_lookup[5 * 4 * 2] = {