ogt_vox: material property _media_type not handled yet

[mgerhardy]

It looks like there are some new or unhandled material properties for blend, glass and cloud materials

material prop	possible values
_media_type	_scatter, _emit, _sss (default is absorb - if _media_type is not available)

value	desc
_sss	Subsurface Scattering Media
_scatter	Scatter Media
_emit	Emissive Media

// media type for blend, glass and cloud materials
enum ogt_media_type {
  ogt_media_type_absorb,  // Absorb media
  ogt_media_type_scatter, // Scatter media
  ogt_media_type_emit,    // Emissive media
  ogt_media_type_sss,     // Subsurface scattering media
};

Example files:

• https://github.com/vengi-voxel/vengi/blob/master/data/tests/test_material.vox
• untitled.zip

(I still wonder whether it makes sense to give low level access to the dict data in the scene to support future values)