instancer: remove unused regions from VarRegionList

if the original VarStore had any regions in VarRegionList that wasn't
even referenced in any VarData VarRegionIndex, this makes sure we
remove those as well from VarRegionList (and remap the VarRegionIndex
accordingly)

Lib/fontTools/varLib/instancer.py

@@ -297,6 +297,8 @@ class _TupleVarStoreAdapter(object):
                 )
         regionList = builder.buildVarRegionList(self.regions, self.axisOrder)
         itemVarStore = builder.buildVarStore(regionList, varDatas)
+        # remove unused regions from VarRegionList
+        itemVarStore.prune_regions()
         return itemVarStore