gftools gen-spac: support distinct user axis values

[RickyDaMa]

For a project, we want to use 200 as the max, but have the axis read as 100. This adds that, by adding two optional flags which overwrite the fvar axis min/max if provided

I will work on rebasing #1214 on top of this Update: done

Open to bikeshedding flag names & help strings

[m4rc1e]

Can you explain why you need this? I cannot work out why you'd need the SPAC axis min and max to have different vals.

[RickyDaMa]

Essentially we want the axis max to be 100 (--user-max) - this is to follow the convention outlined in the axis registry (link). We also want the axis to correspond to a ~20% increase in the spacing, which is the max value of 200. Maybe it's simpler to show as a designspace snippet:

<axis tag="SPAC" name="Spacing" minimum="0" maximum="100" default="0">
    <map input="100" output="200"/>
    <map input="0" output="0"/>
</axis>

cc @MariannaPaszkowska