* Add support for KHR_materials_volume
* Add ubershader support for KHR_materials_volume
* Update readmes
* User highp on anything that affects UVs
* Add missing precision qualifiers
* Fix transmission
* Add support for sheenColor and sheenRoughness
This work is necessary to support the glTF extension KHR_materials_sheen.
This change effectively adds the specular lobe from the cloth material
model to the base material model. The cloth model remains useful for
its extra subsurface color feature but also because it's cheaper.
* Add support for KHR_materials_sheen to gltfio
* Update documentation
* Document default shading values
The equality operator for MaterialKey was missing some fields (also, its
ordering was inconsistent with the struct), which caused faulty behavior
in the material cache.
I ran into this bug when testing against a Blender-exported model that
has some materials with texture transforms, and some without.
This chooses type=THIN, mode=SCREENSPACE, and blending=TRANSPARENT.
Due to sampler overload, we print a friendly error message and enable a
fallback if the user tries to simultaneously enable ubershader mode, the
clearcoat extension, and the transmission extension.
