support relative coordinates

[SwissalpS]

The core developers are adding relative coordinate support to commands:
~x ~-y z ==> x and y coordinates are relative to current and z coordinate is absolute.

I think they also added a convenience function which could be used to support
this entry type in the JD formspec.

[BuckarooBanzay]

I think they also added a convenience function which could be used to support
this entry type in the JD formspec.

If that's the case it would be pretty easy to implement

[SwissalpS]

backward compat would make this a little more complicated unless jd's min MT version is bumped up.

[S-S-X]

Just change input keys rel_x, rel_y, rel_z to easily keep everything compatible.

[SwissalpS]

I was thinking of not adding more fields to formspec, allowing user to prepend a number with '~' to indicate that it is a relative coordinate.

[S-S-X]

convenience function

Thinking about this a bit I believe this is one of those "convenience" things you probably should avoid and just implement very basic math yourself.
I mean it sounds kinda like these useless packages: https://www.npmjs.com/package/is-odd big framework built around return (n % 2) == 1;