Avoid unnecessary shared_ptr copies and simplify static initialization

[vittorioromeo]

Untested, but the behaviour should be the same unless I am missing something. Basically, I've changed some auto to auto& to avoid an unnecessary std::shared_ptr copy, and also simplified the initialization of some static function-scope variables, as I don't think that an extra check was needed (again, unless I am missing something).

Will likely have no significant impact on performance but if many std::shared_ptr copies were being unnecessarily made per-frame then it might actually reduce CPU usage a bit.

Also, (again -- I might be missing something), all of these ::get() functions maybe do not even need shared pointers at all. E.g. how about:

RE8VR& RE8VR::get() {
    static RE8VR inst;
    return inst;
}

?

[praydog]

Tested it for a bit in MHRise with a bunch of scripts & then with VR loaded, did not notice any bugs. Thanks for the PR.

I'm not super against refactoring the ::get() functions to return a reference to a static as long as nothing is broken still.