VertexManagerBase: Make CreateNativeVertexFormat return a unique_ptr

Much safer as opposed to just returning raw allocated memory.

Source/Core/VideoBackends/D3D12/VertexManager.h

@@ -17,7 +17,9 @@ public:
   VertexManager();
   ~VertexManager();
 
-  NativeVertexFormat* CreateNativeVertexFormat(const PortableVertexDeclaration& vtx_decl) override;
+  std::unique_ptr<NativeVertexFormat>
+  CreateNativeVertexFormat(const PortableVertexDeclaration& vtx_decl) override;
+
   void CreateDeviceObjects() override;
   void DestroyDeviceObjects() override;