GLTF::Object should have a virtual destructor?

[ziriax]

It is a common C++ design practice to always make destructors of base classes virtual, so that deleting a pointer to a base instance will call the derived constructor.

Do you guys agree? Then I'll make a patch request.

This would make fixing memory leaks in Maya2glTF easier by just keeping a vector of <GLTF::Object*>

[lasalvavida]

Seems like a good idea to me, happy to do a review of a PR.