KHR_material_ior: missing extension support

[bhouston]

This project is currently missing support for the recent KHR_materials_ior extension.

Unfortunately there is no single test model for this extension, rather it appears to always be used in combination with other extensions, such as KHR_materials_volume. Here is an example of it in combination with KHR_volume and you can see in the STELLAR render the ior is not 1, but in the V-Ray result the IOR is 1, thus no background distortion:

Reference image from Daussaul STELLAR:

Current V-Ray results:

Reference model: khronos-DragonAttenuation

[jasondavies]

I'm going to work on this.