[Idea] enhance the Anchor spec to allow ignoring x or y

[justvanrossum]

Suggestion from @khaledhosny in #90:

An idea for future UFO versions; either specify that anchors can have x or y omitted and then they would be displayed as lines instead of points, or define a new structure that would have such property. This can be used for ligature caret as well as MATH italic correction and top accent positions (both are x only), and any potential new property that would need this.

I think this idea has merit.

(While we're at it, perhaps Anchors need an optional "type" attribute, a (standardized) string to further enhance to applicability of Anchors to solve problems. See this comment of how FontForge uses such a concept. However, perhaps we don't need this at all, and naming conventions are enough. If anyone believes a type attribute would substantially increase the usability of Anchors: please open a new issue.)

[khaledhosny]

There is already a 3 years old issue for anchor type and other attributes #32

[justvanrossum]

Closing, let's keep the discussion in #32.

[justvanrossum]

Sorry, that was about type. Reopening for the horizontal/vertical anchors.

[justvanrossum]

Horizontal/vertical anchors could be done by declaring the x and y attributes optional. A missing y means this is a vertical anchor, a missing x means it's a horizontal anchor. It should probably not be legal to omit both.

[drj11]

i independently implemented x-only anchors in my own font format before coming across UFO, so I'm +1 for this.