Deprecate the variant API

[mwestphal]

Is your feature request related to a problem? Please describe.
In the libf3d, there is three way to access the options, string API, variant API, struct API.
However, the variant API is a remnant of F3D 2 and is not really needed.
It also adds complexity to the code.

Lets discuss and see if we should keep it around.
If not, lets deprecate it cleanly.