[screen-orientation] The orientation type 'Natural' should only lock to x-primary (#83)

The following definition has contradictions, leading to confusion as to what is 'natural':
> natural: Append portrait-primary, portrait-secondary, landscape-primary or landscape-secondary to orientations such as the associated screen orientation angle is 0.

The angle shall never be `0` on a -secondary position. The point of `natural` is to be a unique `0` angle with a `180` reverse angle. Allowing an implementation to lock to `x-secondary` with `natural` would reduce natural to an orientation as opposed an angle, which is the initial point of `natural` isn't it?

It should definitely not be the same as `any`. The platform convention is most likely to do natural with an 'any' lock here anyway. 


---
Reply to this email directly or view it on GitHub:
https://github.com/w3c/screen-orientation/issues/83