Also renamed AxisTent => AxisTriple because I think "tent" is more appropriate to visualize master supports (i.e. a triangle with a peak in the middle), here it's simply the new desired min, default, max values of an axis, or the axis limits.
The public facing instantiateVariableFont continues to accept a hybrid map of axis tags to single values or 2- (and now also 3-) tuples, these are normalized to an AxisLimits mapping that contains only AxisTriples and passed around to the internal methods.
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