4294967295 (0xFFFFFFFF in decimal form) is not very memorable.
Still, using hex notation for all VarIndexBases would make the non-default values less readable (when interpreted as an index into the DeltaSetIndexMap array, decimal makes more sense).
Since 0xFFFFFFFF means 'no variation data', it makes sense to omit it from the ttx dump and write an empty <VarIndexBase/> element with no value.
We also allow to build Var tables without needing to pass "VarIndexBase": 0xFFFFFFFF in the source dict.
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