VertexData description does not specify delta encoding

[nitrologic]

Reading the document in this repos and here there is no mention of the fact that u, v, and height arrays are both delta and zigzag encoded.

According to the source, it is evident that CesiumTerrainProvider interprets the values as deltas:

       var i;
        var u = 0;
        var v = 0;
        var height = 0;

        function zigZagDecode(value) {
            return (value >> 1) ^ (-(value & 1));
        }

        for (i = 0; i < vertexCount; ++i) {
            u += zigZagDecode(uBuffer[i]);
            v += zigZagDecode(vBuffer[i]);
            height += zigZagDecode(heightBuffer[i]);

            uBuffer[i] = u;
            vBuffer[i] = v;
            heightBuffer[i] = height;
        }

[pjcozzi]

Good eye, thanks @nitrologic! @tfili could you please make a spec "bug fix?"

[tfili]

Thanks @nitrologic. I've opened a PR to update the spec.