* Add designspace lib key to specify a custom feature tag for feature variations, as discussed in #2079
* Added comment for the lib key
* if the lib key is present, the 'processing' flag should be ignored completely
'overlap' parameter in instantiateVariableFont was a bool, now it is a tri-state IntEnum, with value 2 meaning 'remove the overlaps'. The old bool False/True (0/1) values continue to work like before.
Also added a new --remove-overlaps commandline option to fonttools varLib.instancer script.
Fix for #1985:
* ensure that the AxisNameID in the STAT table is not less than 256. This needed an additional argument to the addMultiLingualName() name table method.
* fvar axis name IDs must also not be less than 256, just like STAT axis names.
* Allow feature variations to be active across the entire space
* Add test with empty condition set
* Skip initial box instead of removing it
* Correct comment
Note UI change : `fonttools varLib.models` now takes prefixed options `-d` or `-l` instead of guessing the intended feature from the number of arguments.
We have a number of command line tools which are somewhat opaque. (varLib.models in particular was very confusing.) This ensures that they all use argparse to have a consistent interface, and all have --help documentation which at least details their parameters, and hopefully therefore gives more of a clue about what they do. Those which use logging have had a command-line logging parameter added.
This adds a `help` verb (and `--help` option) to the `fonttools` command line tool. Submodules will be listed in the help text if they have an importable `main` function with a docstring, and `main`'s docstring will be used as the one-line description for the help text.
Nothing clever, if the source font has a CFF2 table use it as if it were
a CFF table (i.e. non-variable, not sure what would happen in the source
CFF2 was variable already).
I use the term 'location' for map of {axis_tag: float} coordinates, 'axisRanges'
for a map of {axis_tag: Tuple[float, float]} ranges, and 'axisLimits' to include
either single-float coordinates or range tuples.
Even if HVAR no longer contains any variations, it's better to keep it because
otherwise one would have to check the glyphs' phantom points to confirm that
the advance widths (or heights for VVAR) don't vary
