…axis min/max

but only for axes that are un-mapped, since virtual master coordinates have no means of mapping {user:design} so are assumed un-mapped (whereas axis min/def/max must be in user-space).

with default glyphs_multivalued=False, we end up only storing one 'Virtual Master' param; also we want to read/write these only when dealing with GSFont, not also GSFontMaster

…params

The test font 'IntermediateLayer.glyphs' has a couple of glyphs that use virtual masters to control the cap height via a 'CPHT' axis.
It contains no Axis Mappings nor Axis Location parameters, the axis coordinates are intented to be un-mapped (user==design). The virtual master custom parameters are expected to be taken into account when computing the CPHT min and max values.

somehow it fails while installing skia-pathops, trying to build from source. Probably we only need to bump requirements-dev.txt but I want to do that separately

in absence of explicit Axis Mappings or Axis Location, we infer wght/wdth user-spacelocations from the instances; however if the axis is un-mapped or resolves in an identity (no-op) mapping, there's no reason to prefer the instance-based mapping over the similarly identity/no-op mapping associated with the masters; actually it's possible that instances do not cover all the masters and thus by preferring the instances' mapping over the masters' we might produce incorrect axis min/max.
So, only use the inferred instance-based mapping when it does something interesting, otherwise keep using the master (identity) mapping.