Mirrors the C++23 <utility> function, std::to_underlying

We need to copy padding in most of these cases, and the objects are trivially copyable; however, BitField prevents trivial copy-assignment.

Rather than makring some parts of VertexLoaderManager dirty in some places and some in others, do it all in VideoState. Also, since CPState no longer contains pointers/non-CP data after d039b1bc0dfaba2a88036468b6d971bb1d7e463d, we can just use p.Do on it instead of manually saving each field.