Fix spec valence start
Fix spec valence start

Reference:
https://github.com/KhronosGroup/glTF/issues/1114#issuecomment-350043164

louquillio/draco,google/draco,google/draco,google/draco,louquillio/draco,louquillio/draco,louquillio/draco,google/draco,google/draco,google/draco,louquillio/draco,google/draco,louquillio/draco