Support for more glTF material extensions.

[StreetwareGames]

Is your feature request related to a problem? Please describe.

Hello, I don't know if Filament is still under active development, but a couple of new glTF extensions dropped recently:

https://github.com/KhronosGroup/glTF/tree/main/extensions/2.0/Khronos/KHR_materials_dispersion
https://github.com/KhronosGroup/glTF/tree/main/extensions/2.0/Khronos/KHR_materials_iridescence
https://github.com/KhronosGroup/glTF/tree/main/extensions/2.0/Khronos/KHR_materials_diffuse_transmission

These three seem reasonably easy to add (given that they include sample code) and would make Filament a more compliant glTF renderer.

Describe the solution you'd like
support for three new glTF material extensions.

Describe alternatives you've considered
None

OS and backend
not backend specific

[romainguy]

Filament is under active development and while we would like to add support for those extensions, it's difficult to prioritize them as no one has been asking for them. We do have a working version of _dispersion and _diffuse_transmission somehwere though.

[pixelflinger]

Filament is under active development and while we would like to add support for those extensions, it's difficult to prioritize them as no one has been asking for them. We do have a working version of _dispersion and _diffuse_transmission somehwere though.

We're also running out of samplers, so some features can't be implemented or would only be supported on FEATURE_LEVEL_3. It's not been a priority.