ogt_vox: added _ri material support

[mgerhardy]

ior is ri - 1.0 (according to VOX4U (unreal 5 plugin)

some of the found values in existing vox files:

key: _ri, value: 1.01 key: _ri, value: 1.02 key: _ri, value: 1.03 key: _ri, value: 1.04 key: _ri, value: 1.09 key: _ri, value: 1.16 key: _ri, value: 1.2 key: _ri, value: 1.3 key: _ri, value: 1.33 key: _ri, value: 1.48 key: _ri, value: 1.52 key: _ri, value: 1.53 key: _ri, value: 1.66 key: _ri, value: 1.71 key: _ri, value: 1.82 key: _ri, value: 1.94 key: _ri, value: 2 key: _ri, value: 2.17 key: _ri, value: 2.33