Prequantization does not preserve additional dimensions.

[mbostock]

It only preserves x and y.

Note that postquantization, as in topojson.quantize and topojson.untransform, does the right thing. It’s just delta.js that’s broken. Well actually I realized there’s a small bug there, too: topojson/topojson-client#12

Moved from topojson/topojson#245.