Implement import & export support for "PBR extensions" in .mtl files
(T101029, also fixes T86736).
Newly supported parameters:
- Roughness (Pr, map_Pr)
- Metallic (Pm, map_Pm)
- Sheen (Ps, map_Ps)
- Clearcoat thickness (Pc) and roughness (Pcr)
- Anisotropy (aniso) and rotation (anisor)
- Transmittance (Tf / Kt)
Exporter has an option to enable these additional PBR parameters
export; defaults to off since not all software understands that.
Exporter UI tweaked and all material-related options were put into
their own separate box.
Added/extended test files in Subversion repository for test coverage.
a99a62231e