VertexManagerBase: Make CreateNativeVertexFormat return a unique_ptr

Much safer as opposed to just returning raw allocated memory.

Source/Core/VideoBackends/Software/SWVertexLoader.cpp

@@ -33,10 +33,10 @@ public:
   void SetupVertexPointers() override {}
 };
 
-NativeVertexFormat*
+std::unique_ptr<NativeVertexFormat>
 SWVertexLoader::CreateNativeVertexFormat(const PortableVertexDeclaration& vtx_decl)
 {
-  return new NullNativeVertexFormat(vtx_decl);
+  return std::make_unique<NullNativeVertexFormat>(vtx_decl);
 }
 
 SWVertexLoader::SWVertexLoader() : LocalVBuffer(MAXVBUFFERSIZE), LocalIBuffer(MAXIBUFFERSIZE)