Blender Git Loki

Cleanup: C++ code style for Ghost-XR

* Avoid deep copy of vectors (technically more than a cleanup).
* Use `std::make_unique` for allocating unique pointers, rather than
manual `new`.
* Use `std::optional` for optional by-value return values, rather than
C-style `bool` to indicate success + return-argument.
* Use references rather than pointers for non-optional arguments.
* Avoid manual `new`/`delete`. Use `std::unique_ptr` for local scope
bound lifetime.
* Use C++ `nullptr` rather than C's `NULL`.
* Remove unnecessary friend declaration.

These changes are generally considered good practise and move us more to
a "modern C++" style. We can still go much further of course.
See https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines.