Default thickness value

[emackey]

Hi, following #1646 in the updated "glTF Settings" node, the "thickness" value should default to zero, not one. This matches glTF's own default thicknessFactor of zero.

I think it's appropriate to write KHR_materials_volume on export when (a) thickness is non-zero or textured, and (b) transmission is non-zero or textured. This can have an effect even in cases where absorption/attenuation/density is not being used on the volume.