The setup before this commit violates the C++ standard.
If two classes with non-standard constructors and/or
destructors are in a union their constructors and/or
destructors have to be called explicitly when
switching between the value types.

Using a union is also unsafe as it's illegal to
access the "wrong" of the two members. But the API
doesn't have a way to tell which member is the active.

The new solution doesn't use unions but allows clients
to safely access even the wrong value.

Accessing non-active members in unions is undefined
behavior. Additionally the C++ Standard doesn't
support anonymous structs. [1]
This puts the current implementation of TVec3 and
TVec4 square into undefined behavior territory.

Removing the unions and anonymous structs is easy
and fixes all problematic paths.

[1]: https://stackoverflow.com/questions/2253878/why-does-c-disallow-anonymous-structs