[OTVar] hidden axis flag

Hi John -

I think that we're in agreement on this. From my perspective as both a designer and a 'tool maker' (in that I want to build a UI for working with variable fonts into the CMS or other web-based UI) I think it's crucial to be able to detect what's available in a particular font, what's advisable to allow users to change (i.e. the flags Peter has described), and what value ranges make sense in order to present a UI that is understandable and usable.

I admit that I'm coming at this mainly from a the perspective of a web-centric designer, thinking about web-based implementations. But I suspect that this may provide a useful platform in which to prototype and test UI/interaction patterns faster and more widely than in traditional software development. This may be a real benefit in encouraging exploration of what may be ultimately more successful in any other kind of implementation.