Currently, addFeatureVariationsRaw always deletes existing GSUB.FeatureVariations and overwrites them with the newly built records.
It doesn't have to, though. If the features for which we are adding feature variations are not already "variable", we can simply append the DS-rules-generated records to the FeatureVariations.
We raise an error if the existing FeatureVariations already reference the same feature tags used for the DS rules, as that could generate ambiguity/undefined logic.
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