Do not round them to integer, but let the caller do the rounding immediately before adding them to the default instance (or just before compiling the binary table as with glyf).
This ensures that the glyphs' left sidebearings are calculated in the same way as they were by varLib.mutator.
If we round deltas too early, then we may get off-by-one differences.
See the glyf table setCoordinates method where left sidebearings are computed.
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